docs(pubsub): add unwrapped subscription sample

[alevenberg]

Part of #11720

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This change is 

[snippet-bot]

Here is the summary of changes.

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• google/cloud/pubsub/samples/samples.cc:380, tag pubsub_create_unwrapped_push_subscription

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[codecov]

Codecov Report

Patch coverage: 82.22% and no project coverage change.

Comparison is base (25023b8) 93.58% compared to head (4118ef3) 93.59%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12090   +/-   ##
=======================================
  Coverage   93.58%   93.59%           
=======================================
  Files        2012     2012           
  Lines      176074   176115   +41     
=======================================
+ Hits       164785   164827   +42     
+ Misses      11289    11288    -1     

Impacted Files	Coverage Δ
google/cloud/pubsub/samples/samples.cc	87.08% <74.07%> (-0.01%)	⬇️
google/cloud/pubsub/subscription_builder.h	96.23% <90.90%> (-0.26%)	⬇️
google/cloud/pubsub/subscription_builder_test.cc	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[alevenberg]

Submit on 07/17/23