fix(pubsub): Change AsyncReadWriteStreamAuth to be usable with unique_ptr

[dpcollins-google]

Also fix a bug where if it was cancelled while auth was being set up, neither the spawning context nor the created stream would be cancelled.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7692 (1f84b47) into main (6557aba) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7692   +/-   ##
=======================================
  Coverage   95.29%   95.29%           
=======================================
  Files        1254     1254           
  Lines      113589   113617   +28     
=======================================
+ Hits       108241   108276   +35     
+ Misses       5348     5341    -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	97.91% <100.00%> (+6.48%)	⬆️
...loud/internal/async_read_write_stream_auth_test.cc	97.14% <100.00%> (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 11 more

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[google-cloud-cpp-bot]

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[coryan]

LGTM, but the robots are still unhappy.

[google-cloud-cpp-bot]

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Addressed

[google-cloud-cpp-bot]

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[coryan]

LGTM. Thanks!