feat(common): mock for AsyncStreamingReadWriteRpc

[coryan]

This class is part of the public API for some *Connection and *Client classes. Applications may need to mock them, we should provide the mock.

I also refactored existing mocks to a single place.

Part of the work for #9134

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This change is 

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (669b751) 93.59% compared to head (45ad3cf) 93.59%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #12950    +/-   ##
========================================
  Coverage   93.59%   93.59%            
========================================
  Files        2067     2068     +1     
  Lines      180575   180686   +111     
========================================
+ Hits       169003   169110   +107     
- Misses      11572    11576     +4     

Files	Coverage Δ
...ration_tests/tests/mock_golden_kitchen_sink_stub.h	77.27% <ø> (+2.27%)	⬆️
...d/internal/async_read_write_stream_logging_test.cc	100.00% <ø> (ø)
...d/internal/async_read_write_stream_tracing_test.cc	100.00% <ø> (ø)
.../cloud/mocks/mock_async_streaming_read_write_rpc.h	100.00% <ø> (ø)
...e/cloud/pubsub/internal/subscriber_stub_factory.cc	100.00% <100.00%> (ø)
...ud/pubsublite/internal/partition_publisher_test.cc	99.24% <ø> (ø)
...l/resumable_async_streaming_read_write_rpc_test.cc	99.87% <ø> (ø)
...ud/pubsub/internal/subscriber_stub_factory_test.cc	96.03% <96.03%> (ø)

... and 6 files with indirect coverage changes

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