Work towards invocation-side security.

src/python/src/grpc/_adapter/_c_test.py

src/python/src/grpc/_adapter/_channel.c

src/python/src/grpc/_adapter/_client_credentials.c

src/python/src/grpc/_adapter/_face_test_case.py

src/python/src/grpc/_adapter/_links_test.py

src/python/src/grpc/_adapter/_lonely_rear_link_test.py

src/python/src/grpc/_adapter/_low_test.py

src/python/src/grpc/_adapter/rear.py

src/python/src/grpc/early_adopter/_reexport.py

src/python/src/grpc/early_adopter/implementations.py

src/python/src/grpc/framework/assembly/implementations.py