Add various options to verify ssl/tls client cert including letting the
application handle the authentication.
diff --git a/gRPC.podspec b/gRPC.podspec
index 7ede97d..859c2c9 100644
--- a/gRPC.podspec
+++ b/gRPC.podspec
@@ -323,6 +323,7 @@
                       'include/grpc/impl/codegen/sync_win32.h',
                       'include/grpc/impl/codegen/time.h',
                       'include/grpc/grpc_security.h',
+                      'include/grpc/grpc_security_constants.h',
                       'include/grpc/census.h',
                       'src/core/lib/channel/channel_args.c',
                       'src/core/lib/channel/channel_stack.c',