Merge pull request #3075 from jboeuf/credentials_naming_and_cleanup

include/grpc++/credentials.h

include/grpc/grpc_security.h

src/core/security/credentials.c

src/core/security/credentials.h

src/core/security/google_default_credentials.c

src/cpp/client/secure_credentials.cc

src/node/ext/credentials.cc

src/php/ext/grpc/credentials.c

src/python/grpcio/grpc/_adapter/_c/types.h

...python/grpcio/grpc/_adapter/_c/types/client_credentials.c

src/python/grpcio/grpc/_cython/_cygrpc/credentials.pyx

src/python/grpcio/grpc/_cython/_cygrpc/grpc.pxd

src/python/grpcio/grpc/_cython/adapter_low.py

src/python/grpcio_test/grpc_interop/methods.py

src/ruby/ext/grpc/rb_credentials.c

...2end/tests/request_response_with_payload_and_call_creds.c

test/core/security/credentials_test.c

test/core/security/fetch_oauth2.c

test/cpp/client/credentials_test.cc

test/cpp/end2end/end2end_test.cc