feat(spanner): add samples for instance partitions

[varuncnaik]

Add samples for instance partitions.

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[snippet-bot]

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• google/cloud/spanner/samples/samples.cc:610, tag spanner_create_instance_partition

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[scotthart]

/gcbrun

[codecov]

Codecov Report

Attention: Patch coverage is 15.78947% with 16 lines in your changes missing coverage. Please review.

Project coverage is 93.58%. Comparing base (c93210d) to head (be1608e).
Report is 2 commits behind head on main.

Files	Patch %	Lines
google/cloud/spanner/samples/samples.cc	15.78%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14574      +/-   ##
==========================================
- Coverage   93.59%   93.58%   -0.01%     
==========================================
  Files        2318     2318              
  Lines      207162   207181      +19     
==========================================
+ Hits       193887   193899      +12     
- Misses      13275    13282       +7     

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