feat(pubsub): exactly-once delivery

[coryan]

Implement the API to subscribe with exactly-once delivery, make
ExactlyOnceAckHandler public, add the samples, and also add an
integration test.

Part of the work for #9327

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Here is the summary of changes.

You are about to add 2 region tags.

• google/cloud/pubsub/samples/samples.cc:293, tag pubsub_create_subscription_with_exactly_once_delivery
• google/cloud/pubsub/samples/samples.cc:1315, tag pubsub_subscriber_exactly_once

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Google Cloud Build Logs
For commit: 643fea0b9247a67c6e5c09a8332dfd8699e4ea19

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[codecov]

Codecov Report

Merging #9436 (a6f80dc) into main (2e1a647) will increase coverage by 0.00%.
The diff coverage is 98.96%.

@@           Coverage Diff            @@
##             main    #9436    +/-   ##
========================================
  Coverage   94.62%   94.62%            
========================================
  Files        1486     1487     +1     
  Lines      136452   136623   +171     
========================================
+ Hits       129116   129282   +166     
- Misses       7336     7341     +5     

Impacted Files	Coverage Δ
google/cloud/pubsub/exactly_once_ack_handler.cc	100.00% <ø> (ø)
google/cloud/pubsub/exactly_once_ack_handler.h	70.58% <ø> (ø)
...ogle/cloud/pubsub/exactly_once_ack_handler_test.cc	100.00% <ø> (ø)
...ubsub/internal/subscription_concurrency_control.cc	94.02% <ø> (ø)
...pubsub/internal/subscription_concurrency_control.h	100.00% <ø> (ø)
.../internal/subscription_concurrency_control_test.cc	100.00% <ø> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	98.49% <ø> (ø)
google/cloud/pubsub/subscriber_connection.cc	77.46% <84.61%> (-1.33%)	⬇️
...b/integration_tests/subscriber_integration_test.cc	98.57% <100.00%> (+0.24%)	⬆️
...ogle/cloud/pubsub/internal/subscription_session.cc	96.12% <100.00%> (-0.03%)	⬇️
... and 8 more

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Google Cloud Build Logs
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