impl(rest): support LRO operation types without name method

[scotthart]

Unlike Compute, BigQuery LRO Operation types don't have a name method. This PR adds a mechanism to provide a function to get the name from the Operation.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 92.91%. Comparing base (bbb1355) to head (0e82dac).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ud/internal/async_rest_polling_loop_custom_test.cc	91.22%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14924   +/-   ##
=======================================
  Coverage   92.91%   92.91%           
=======================================
  Files        2351     2351           
  Lines      209741   209811   +70     
=======================================
+ Hits       194883   194955   +72     
+ Misses      14858    14856    -2     

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