So we can make the property a dictionary later, by just extending
NSMutableDictionary to conform to the protocol.

It’s a typedef of an anonymous struct.

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

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).