doc(pubsub): add get/delete/rollback schema samples

[alevenberg]

Issue 11720

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This change is 

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• google/cloud/pubsub/samples/samples.cc:921, tag pubsub_get_schema_revision
• google/cloud/pubsub/samples/samples.cc:997, tag pubsub_delete_schema_revision
• google/cloud/pubsub/samples/samples.cc:1019, tag pubsub_rollback_schema

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[coryan]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @alevenberg)

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google/cloud/pubsub/samples/samples.cc line 2067 at r1 (raw file):

  GetSchema(schema_admin, {project_id, avro_schema_id});

  // For testing RollbackSchema, create 2 new schema revisions and rollback to

nit: s/For testing/To test/ ? 🤷 what do I know.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (f4d98f6) 93.79% compared to head (a4371dd) 93.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11848      +/-   ##
==========================================
+ Coverage   93.79%   93.81%   +0.01%     
==========================================
  Files        1838     1838              
  Lines      166384   166441      +57     
==========================================
+ Hits       156067   156152      +85     
+ Misses      10317    10289      -28     

Impacted Files	Coverage Δ
...ogle/cloud/pubsub/samples/pubsub_samples_common.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/samples/samples.cc	87.56% <100.00%> (+0.36%)	⬆️

... and 8 files with indirect coverage changes

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