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/config.m4 b/config.m4
index 74f9ad2..40e827d 100644
--- a/config.m4
+++ b/config.m4
@@ -188,20 +188,28 @@
     src/core/ext/transport/chttp2/transport/writing.c \
     src/core/ext/transport/chttp2/alpn/alpn.c \
     src/core/lib/http/httpcli_security_connector.c \
-    src/core/lib/security/b64.c \
-    src/core/lib/security/client_auth_filter.c \
-    src/core/lib/security/credentials.c \
-    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/json_token.c \
-    src/core/lib/security/jwt_verifier.c \
-    src/core/lib/security/secure_endpoint.c \
-    src/core/lib/security/security_connector.c \
-    src/core/lib/security/security_context.c \
-    src/core/lib/security/server_auth_filter.c \
+    src/core/lib/security/context/security_context.c \
+    src/core/lib/security/credentials/composite/composite_credentials.c \
+    src/core/lib/security/credentials/credentials.c \
+    src/core/lib/security/credentials/credentials_metadata.c \
+    src/core/lib/security/credentials/fake/fake_credentials.c \
+    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/iam/iam_credentials.c \
+    src/core/lib/security/credentials/jwt/json_token.c \
+    src/core/lib/security/credentials/jwt/jwt_credentials.c \
+    src/core/lib/security/credentials/jwt/jwt_verifier.c \
+    src/core/lib/security/credentials/oauth2/oauth2_credentials.c \
+    src/core/lib/security/credentials/plugin/plugin_credentials.c \
+    src/core/lib/security/credentials/ssl/ssl_credentials.c \
+    src/core/lib/security/transport/client_auth_filter.c \
+    src/core/lib/security/transport/handshake.c \
+    src/core/lib/security/transport/secure_endpoint.c \
+    src/core/lib/security/transport/security_connector.c \
+    src/core/lib/security/transport/server_auth_filter.c \
+    src/core/lib/security/util/b64.c \
+    src/core/lib/security/util/json_util.c \
     src/core/lib/surface/init_secure.c \
     src/core/lib/tsi/fake_transport_security.c \
     src/core/lib/tsi/ssl_transport_security.c \
@@ -573,7 +581,18 @@
   PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/iomgr)
   PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/json)
   PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/profiling)
-  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/context)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/credentials)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/credentials/composite)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/credentials/fake)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/credentials/google_default)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/credentials/iam)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/credentials/jwt)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/credentials/oauth2)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/credentials/plugin)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/credentials/ssl)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/transport)
+  PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/security/util)
   PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/support)
   PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/surface)
   PHP_ADD_BUILD_DIR($ext_builddir/src/core/lib/transport)