RFC: Allow REST transport for PubSub Java client.

[sanjivr]

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Supports #2074

This leverages capabilities added in #1162

We're looking for input on whether

• this approach is feasible or not
• the SDK might consider making this generally available after the approach has been ratified and unit tests are included.

[sanjivr]

Note: If the user attempts to enable compression on the JSON transport, should the SDK throw an exception or silently ignore the same as the HttpJsonCallContext does not support compression ?