docs(pubsub): add a subscriber quickstart

[alevenberg]

Create a subscriber executable for the Pub/Sub quickstart

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[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• google/cloud/pubsub/quickstart/subscriber_quickstart.cc:15, tag pubsub_quickstart_subscriber

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[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (8ce34eb) 93.68% compared to head (9c6e9b2) 93.68%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12053   +/-   ##
=======================================
  Coverage   93.68%   93.68%           
=======================================
  Files        2014     2014           
  Lines      175262   175262           
=======================================
+ Hits       164190   164192    +2     
+ Misses      11072    11070    -2     

see 5 files with indirect coverage changes

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[coryan]

You probably need to rebase if you want the new build to pass.