Support test certificates and SSL name override in Obj-C

Allow fixtures to specify slowdown factors, use it for grpc_trace tests

Delay name resolution until we need a channel to be READY

C++ propagation API

Core-supported context propagation

Introduce client-side CallOptions 

It’s a typedef of an anonymous struct.

Add project metadata generation to Python.

And add warning about using custom certificates or name override if not
testing.

Calling python to spawn a python subprocess.

Add StubOptions