…erStreaming support in REST transport

This includes the following changes for HTTP1.1/REST transport:
1) `HttpJsonClientCall` class (with `HttpJsonClientCall.Listener`) mimicking [io.grpc.ClientCall](https://github.com/grpc/grpc-java/blob/master/api/src/main/java/io/grpc/ClientCall.java#L102) functionality.
2) The unary callables are rewritten to be based on `HttpJsonClientCall` flow (similarly to how it is already done in grpc unary calls).
3) Server streaming support for REST transport. The implementation is based on `HttpJsonClientCall` and `HttpJsonClientCall.Listener`, similarly to how grpc streaming is based on `io.grpc.ClientCall` and io.grpc.ClientCall.Listener` respectively.

The extreme similarity between `HttpJsonClient` call and `io.grpc.ClientCall` is intentional and crucial for consistency of the two transports and also intends simplifying creation and maintenance of multi-transport manual wrappers (like [google-ads](https://github.com/googleads/google-ads-java/blob/main/google-ads/src/main/java/com/google/ads/googleads/lib/logging/LoggingInterceptor.java#L68)).

The server streaming abstractions in gax java are all based on the flow control managed by a ClientCall, so having similar set of abstractions in REST transport is necessary to reuse transport-independent portions of streaming logic in gax and maintain identical user-facing streaming surface.

This PR also builds a foundation for the soon-coming [ClientInterceptor](https://github.com/grpc/grpc-java/blob/master/api/src/main/java/io/grpc/ClientInterceptor.java#L42)-like infrastructure in REST transport.

REST-based client-side streaming and bidirectional streaming is not implemented by this PR and most likely will never be due to limitations of the HTTP1.1/REST protocol compared to HTTP2/gRPC.

Note, ideally I need to split this PR at least in two separate ones: 1) HttpJsonClientCall stuff and unary calls based on it in one PR and then 2) server streaming feature in a second PR. Unfortunately only reasonable way to test `HttpJsonClientCall` infrastructure is by doing it from server streaming logic beause most of the complexity introduced in HttpJsonClient call is induced by necessity to support streaming workflow.

…p detection