refactor: compress RetryPolicy class hierarchy

[scotthart]

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 89.79592% with 5 lines in your changes missing coverage. Please review.

Project coverage is 92.91%. Comparing base (ee8c2bb) to head (a3b3383).

Files with missing lines	Patch %	Lines
...v1/internal/golden_kitchen_sink_connection_impl.cc	50.00%	1 Missing ⚠️
...ternal/golden_kitchen_sink_rest_connection_impl.cc	50.00%	1 Missing ⚠️
.../v1/internal/golden_thing_admin_connection_impl.cc	80.00%	1 Missing ⚠️
...nternal/golden_thing_admin_rest_connection_impl.cc	80.00%	1 Missing ⚠️
google/cloud/bigtable/testing/mock_policies.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #14947    +/-   ##
========================================
  Coverage   92.91%   92.91%            
========================================
  Files        2351     2351            
  Lines      210082   209859   -223     
========================================
- Hits       195190   194991   -199     
+ Misses      14892    14868    -24     

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