feat(pubsub): implement per-batch compression

[coryan]

New options to compress published messages before they are put on the
wire. This can reduce bandwidth usage.

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[snippet-bot]

Here is the summary of changes.

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• google/cloud/pubsub/samples/samples.cc:1562, tag pubsub_publish_with_compression

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: df78cef016efb821bf7a53a1fb017878a54c8684

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[codecov]

Codecov Report

Merging #9270 (c4ba1c3) into main (e61b6da) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #9270   +/-   ##
=======================================
  Coverage   94.58%   94.58%           
=======================================
  Files        1462     1462           
  Lines      132540   132596   +56     
=======================================
+ Hits       125363   125421   +58     
+ Misses       7177     7175    -2     

Impacted Files	Coverage Δ
google/cloud/pubsub/internal/default_batch_sink.cc	100.00% <100.00%> (ø)
...e/cloud/pubsub/internal/default_batch_sink_test.cc	97.33% <100.00%> (+0.78%)	⬆️
google/cloud/pubsub/internal/defaults.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/internal/defaults_test.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/samples/samples.cc	92.18% <100.00%> (+0.21%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	97.75% <0.00%> (-0.25%)	⬇️
...torage/examples/storage_service_account_samples.cc	97.64% <0.00%> (+1.17%)	⬆️

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: c4ba1c30e54343f5495eaa5147dac80da060e4dd

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[coryan]

/FYI: @anguillanneuf

[scotthart]

Reviewed 5 of 6 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)