[java-pubsub] Allow user to configure usage of REST Transport

**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.