Surfaced in GRPCCall+Tests.h

Add GRPCHost to store channel config, and to create channels on demand
with that config. GRPCChannels and configs are cached together.

GRPCSecureChannel is now initialized with (nullable) path to a
certificates file and (nullable) name override.

The same mechanism will be used for creating insecure channels, removing
the ability to do it by specifying the HTTP scheme in the address (which
was deemed too subtle for its implications).

They don’t work against the local server.

We don’t use the Tests library target anyway.

And move it to the RxLibrary.

With GRXWriter a subclass of all writers, we can map the requestsWriter directly
without a dumb wrapper, and return GRXImmediateWriter objects where GRXWriter is
expected.