Add a ChannelProvider for use with the emulator

[kamalaboulhosn]

The gcloud Pub/Sub emulator require a Channel that has negotiationType set to NegotiationType.PLAINTEXT. That means one needs to create a ChannelProvider that returns Channels with this set. It would be useful if the client library provided the ability to set this on a ChannelProvider in the library already or a builder that could be used to generate one easily.

[pongad]

Duplicate of #1973