Merge github.com:grpc/grpc into virtuous-velvit-velociraptor

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

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

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

test/core/security/credentials_test.c

test/cpp/interop/client_helper.cc