feat(generator): merge connection options into client options

[devbww]

Store the Connection options so that they may be made available to
merge into the Client options, and then use them to implement the
four polling-policy data elements. (This is the generator version
of #8090.)

Rules of thumb for Options values:

• MergeOptions() LHS should not include the service defaults
• MergeOptions() RHS should include service defaults
• An OptionsSpan should include the service defaults
• stored Options (data members) should include the service defaults

Code can then normally just look at the CurrentOptions(), knowing
that they will include the service defaults. For example, the policy
options will be available. A special exception to this, however, is
when testing the connection layer in isolation (without a client), as
no one has created an OptionsSpan in that case. We could add an
OptionsSpan to the connection-layer calls, but it would be a no-op
in practice. So, tests need to ensure required options are available.

Fixes #8054.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #8158 (0c68fb1) into main (223a6e0) will increase coverage by 0.00%.
The diff coverage is 97.14%.

@@           Coverage Diff           @@
##             main    #8158   +/-   ##
=======================================
  Coverage   94.58%   94.58%           
=======================================
  Files        1320     1322    +2     
  Lines      117829   117838    +9     
=======================================
+ Hits       111451   111461   +10     
+ Misses       6378     6377    -1     

Impacted Files	Coverage Δ
generator/internal/client_generator.cc	29.25% <ø> (ø)
...den/internal/golden_kitchen_sink_connection_impl.h	77.77% <80.00%> (-3.48%)	⬇️
...gration_tests/golden/golden_kitchen_sink_client.cc	87.67% <100.00%> (ø)
...ion_tests/golden/golden_kitchen_sink_connection.cc	40.00% <100.00%> (+1.53%)	⬆️
...tion_tests/golden/golden_kitchen_sink_connection.h	100.00% <100.00%> (ø)
...egration_tests/golden/golden_thing_admin_client.cc	98.00% <100.00%> (ø)
...tion_tests/golden/golden_thing_admin_connection.cc	21.05% <100.00%> (+1.05%)	⬆️
...ation_tests/golden/golden_thing_admin_connection.h	100.00% <100.00%> (ø)
...en/internal/golden_kitchen_sink_connection_impl.cc	90.90% <100.00%> (ø)
...den/internal/golden_thing_admin_connection_impl.cc	94.44% <100.00%> (-0.03%)	⬇️
... and 14 more

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[google-cloud-cpp-bot]

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[devbww]

Don't bother looking at the "regenerated" commits.

[coryan]

Thank you for separating the "generated" commits, made it much easier to review things.

Do you think it would be possible to add some sort of unit test to the generated golden files? We don't write tests for each generated library, and we explain this to ourselves by testing the golden files once. I am Okay we decide to add them in a separate PR.

[dbolduc]

Thanks for doing the fix

[devbww]

Do you think it would be possible to add some sort of unit test to the generated golden files?

Yes, I'll look into it. Thanks.