Refactoring the core security code.

As opposed to a flat directory, we now have the following structure:

- security
  -context
  - credentials
    - composite
    - fake
    - google_default
    - iam
    - jwt
    - oauth2
    - plugin
    - ssl
  - transport
  - util

We have not refactored the test code yet but this PR is already large
enough...
diff --git a/tools/run_tests/sources_and_headers.json b/tools/run_tests/sources_and_headers.json
index f546f3b..6c08a80 100644
--- a/tools/run_tests/sources_and_headers.json
+++ b/tools/run_tests/sources_and_headers.json
@@ -6014,15 +6014,24 @@
     "headers": [
       "include/grpc/grpc_security.h", 
       "include/grpc/grpc_security_constants.h", 
-      "src/core/lib/security/auth_filters.h", 
-      "src/core/lib/security/b64.h", 
-      "src/core/lib/security/credentials.h", 
-      "src/core/lib/security/handshake.h", 
-      "src/core/lib/security/json_token.h", 
-      "src/core/lib/security/jwt_verifier.h", 
-      "src/core/lib/security/secure_endpoint.h", 
-      "src/core/lib/security/security_connector.h", 
-      "src/core/lib/security/security_context.h"
+      "src/core/lib/security/context/security_context.h", 
+      "src/core/lib/security/credentials/composite/composite_credentials.h", 
+      "src/core/lib/security/credentials/credentials.h", 
+      "src/core/lib/security/credentials/fake/fake_credentials.h", 
+      "src/core/lib/security/credentials/google_default/google_default_credentials.h", 
+      "src/core/lib/security/credentials/iam/iam_credentials.h", 
+      "src/core/lib/security/credentials/jwt/json_token.h", 
+      "src/core/lib/security/credentials/jwt/jwt_credentials.h", 
+      "src/core/lib/security/credentials/jwt/jwt_verifier.h", 
+      "src/core/lib/security/credentials/oauth2/oauth2_credentials.h", 
+      "src/core/lib/security/credentials/plugin/plugin_credentials.h", 
+      "src/core/lib/security/credentials/ssl/ssl_credentials.h", 
+      "src/core/lib/security/transport/auth_filters.h", 
+      "src/core/lib/security/transport/handshake.h", 
+      "src/core/lib/security/transport/secure_endpoint.h", 
+      "src/core/lib/security/transport/security_connector.h", 
+      "src/core/lib/security/util/b64.h", 
+      "src/core/lib/security/util/json_util.h"
     ], 
     "language": "c", 
     "name": "grpc_secure", 
@@ -6030,29 +6039,46 @@
       "include/grpc/grpc_security.h", 
       "include/grpc/grpc_security_constants.h", 
       "src/core/lib/http/httpcli_security_connector.c", 
-      "src/core/lib/security/auth_filters.h", 
-      "src/core/lib/security/b64.c", 
-      "src/core/lib/security/b64.h", 
-      "src/core/lib/security/client_auth_filter.c", 
-      "src/core/lib/security/credentials.c", 
-      "src/core/lib/security/credentials.h", 
-      "src/core/lib/security/credentials_metadata.c", 
-      "src/core/lib/security/credentials_posix.c", 
-      "src/core/lib/security/credentials_win32.c", 
-      "src/core/lib/security/google_default_credentials.c", 
-      "src/core/lib/security/handshake.c", 
-      "src/core/lib/security/handshake.h", 
-      "src/core/lib/security/json_token.c", 
-      "src/core/lib/security/json_token.h", 
-      "src/core/lib/security/jwt_verifier.c", 
-      "src/core/lib/security/jwt_verifier.h", 
-      "src/core/lib/security/secure_endpoint.c", 
-      "src/core/lib/security/secure_endpoint.h", 
-      "src/core/lib/security/security_connector.c", 
-      "src/core/lib/security/security_connector.h", 
-      "src/core/lib/security/security_context.c", 
-      "src/core/lib/security/security_context.h", 
-      "src/core/lib/security/server_auth_filter.c", 
+      "src/core/lib/security/context/security_context.c", 
+      "src/core/lib/security/context/security_context.h", 
+      "src/core/lib/security/credentials/composite/composite_credentials.c", 
+      "src/core/lib/security/credentials/composite/composite_credentials.h", 
+      "src/core/lib/security/credentials/credentials.c", 
+      "src/core/lib/security/credentials/credentials.h", 
+      "src/core/lib/security/credentials/credentials_metadata.c", 
+      "src/core/lib/security/credentials/fake/fake_credentials.c", 
+      "src/core/lib/security/credentials/fake/fake_credentials.h", 
+      "src/core/lib/security/credentials/google_default/credentials_posix.c", 
+      "src/core/lib/security/credentials/google_default/credentials_win32.c", 
+      "src/core/lib/security/credentials/google_default/google_default_credentials.c", 
+      "src/core/lib/security/credentials/google_default/google_default_credentials.h", 
+      "src/core/lib/security/credentials/iam/iam_credentials.c", 
+      "src/core/lib/security/credentials/iam/iam_credentials.h", 
+      "src/core/lib/security/credentials/jwt/json_token.c", 
+      "src/core/lib/security/credentials/jwt/json_token.h", 
+      "src/core/lib/security/credentials/jwt/jwt_credentials.c", 
+      "src/core/lib/security/credentials/jwt/jwt_credentials.h", 
+      "src/core/lib/security/credentials/jwt/jwt_verifier.c", 
+      "src/core/lib/security/credentials/jwt/jwt_verifier.h", 
+      "src/core/lib/security/credentials/oauth2/oauth2_credentials.c", 
+      "src/core/lib/security/credentials/oauth2/oauth2_credentials.h", 
+      "src/core/lib/security/credentials/plugin/plugin_credentials.c", 
+      "src/core/lib/security/credentials/plugin/plugin_credentials.h", 
+      "src/core/lib/security/credentials/ssl/ssl_credentials.c", 
+      "src/core/lib/security/credentials/ssl/ssl_credentials.h", 
+      "src/core/lib/security/transport/auth_filters.h", 
+      "src/core/lib/security/transport/client_auth_filter.c", 
+      "src/core/lib/security/transport/handshake.c", 
+      "src/core/lib/security/transport/handshake.h", 
+      "src/core/lib/security/transport/secure_endpoint.c", 
+      "src/core/lib/security/transport/secure_endpoint.h", 
+      "src/core/lib/security/transport/security_connector.c", 
+      "src/core/lib/security/transport/security_connector.h", 
+      "src/core/lib/security/transport/server_auth_filter.c", 
+      "src/core/lib/security/util/b64.c", 
+      "src/core/lib/security/util/b64.h", 
+      "src/core/lib/security/util/json_util.c", 
+      "src/core/lib/security/util/json_util.h", 
       "src/core/lib/surface/init_secure.c"
     ], 
     "third_party": false,