feat(generator): create retry policy samples

[coryan]

This change is 

[codecov]

Codecov Report

Patch coverage: 94.25% and project coverage change: -0.01 ⚠️

Comparison is base (bbf1134) 93.66% compared to head (0d6b7aa) 93.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11930      +/-   ##
==========================================
- Coverage   93.66%   93.66%   -0.01%     
==========================================
  Files        1838     1838              
  Lines      166523   166694     +171     
==========================================
+ Hits       155978   156137     +159     
- Misses      10545    10557      +12     

Impacted Files	Coverage Δ
...ogle/cloud/pubsub/samples/schema_client_samples.cc	98.36% <ø> (+1.39%)	⬆️
...e/cloud/pubsublite/samples/admin_client_samples.cc	98.36% <ø> (+1.39%)	⬆️
...d/pubsublite/samples/topic_stats_client_samples.cc	98.36% <ø> (+1.39%)	⬆️
generator/internal/sample_generator.cc	75.92% <66.66%> (-3.57%)	⬇️
...n/v1/samples/golden_kitchen_sink_client_samples.cc	98.24% <100.00%> (+1.27%)	⬆️
...lden/v1/samples/golden_rest_only_client_samples.cc	98.24% <100.00%> (+1.27%)	⬆️
...en/v1/samples/golden_thing_admin_client_samples.cc	98.24% <100.00%> (+1.27%)	⬆️

... and 4 files with indirect coverage changes

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