Allow user to configure usage of REST Transport

[sanjivr]

Is your feature request related to a problem? Please describe.

The PubSub Java SDK does not work when configured with InstantiatingHttpJsonChannelProvider.
It only supports InstantiatingGrpcChannelProvider. The Publisher implementation
provided by the SDK is tightly coupled with a Grpc transport.

Describe the solution you'd like

Update the Publisher and Subscriber implementations to support InstantiatingHttpJsonChannelProvider

Describe alternatives you've considered

None

Additional context

• We have a requirement for on-premise clients to access PubSub only through a TLS terminating proxy
  which performs additional validation on request attributes to limit traffic to just allowlisted GCP projects
  or pubsub topics
  • In the TLS terminating mode, the proxy doesn't support grpc. As such all clients leveraging the official PubSub Java SDK are unable to utilize this TLS terminated proxy approach.

[sanjivr]

We have patched the Publisher implementation to support InstantiatingHttpJsonChannelProvider
and verified that the same works for our use case.

#2075 illustrates the patch.

Please let us know whether there are any limitations in the implementation approach and
if it can be considered for general availability.