feat(common): GUAC for async read-write streams

[coryan]

This change is 

[google-cloud-cpp-bot]

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For commit: 00fc96c41729be265e65e4b00884684ac2982aa0

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[codecov]

Codecov Report

Merging #7442 (4eacd41) into main (6137b15) will decrease coverage by 0.00%.
The diff coverage is 91.76%.

@@            Coverage Diff             @@
##             main    #7442      +/-   ##
==========================================
- Coverage   93.64%   93.64%   -0.01%     
==========================================
  Files        1362     1364       +2     
  Lines      118253   118330      +77     
==========================================
+ Hits       110735   110805      +70     
- Misses       7518     7525       +7     

Impacted Files	Coverage Δ
google/cloud/pubsub/internal/subscriber_auth.h	100.00% <ø> (ø)
google/cloud/pubsub/internal/subscriber_logging.h	100.00% <ø> (ø)
...oogle/cloud/pubsub/internal/subscriber_metadata.cc	100.00% <ø> (ø)
...le/cloud/pubsub/internal/subscriber_round_robin.cc	100.00% <ø> (ø)
...gle/cloud/pubsub/internal/subscriber_round_robin.h	100.00% <ø> (ø)
google/cloud/pubsub/internal/subscriber_stub.h	100.00% <ø> (ø)
...ogle/cloud/internal/async_read_write_stream_auth.h	82.85% <82.85%> (ø)
...loud/internal/async_read_write_stream_auth_test.cc	97.14% <97.14%> (ø)
...ternal/streaming_subscription_batch_source_test.cc	99.23% <100.00%> (ø)
google/cloud/pubsub/internal/subscriber_auth.cc	100.00% <100.00%> (ø)
... and 10 more

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 4eacd41a42db80207bee36e60a2b0df1a4dea86d

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