test(pubsub): add create_pull_subscription sample

[haphungw]

Implement create_pull_subscription and include an integration test.

For #3017

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.02%. Comparing base (76753ba) to head (0e160f8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4583      +/-   ##
==========================================
+ Coverage   95.01%   95.02%   +0.01%     
==========================================
  Files         195      195              
  Lines        7456     7456              
==========================================
+ Hits         7084     7085       +1     
+ Misses        372      371       -1     

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