Add CallCredentials class to Ruby wrapping code

src/ruby/ext/grpc/rb_call.c

src/ruby/ext/grpc/rb_call.h

src/ruby/ext/grpc/rb_call_credentials.c

src/ruby/ext/grpc/rb_call_credentials.h

src/ruby/ext/grpc/rb_channel_credentials.c

src/ruby/ext/grpc/rb_grpc.c