test(pubsub): add quickstart_subscriber sample

[haphungw]

Implement quickstart_subscriber and include an integration test.

For #3017

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.01%. Comparing base (af854d3) to head (daef290).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4572   +/-   ##
=======================================
  Coverage   95.01%   95.01%           
=======================================
  Files         195      195           
  Lines        7449     7449           
=======================================
  Hits         7078     7078           
  Misses        371      371           

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[dbolduc]

Thanks!

I just have nits, many of which are opinions. Feel free to push back.