feat(common): add ability to supply a user-run CQ to gRPC options

[dbolduc]

Fixes #7346

I went with GrpcCompletionQueueOption as the name.

I went with GrpcCompletionQueueOption > GrpcBackgroundThreadsFactoryOption > GrpcBackgroundThreadPoolSizeOption. (I put the CQ option first in case we ever deprecate the factory option.)

For testing the precedence, we have a > b > c so I just checked that a > b && b > c

I did not propagate the test for the option into things like Spanner, which tests other options from GrpcOptionList. We should think about how to test common options that show up in all of our libraries. Do we want to duplicate a common test N times? maybe.

Bigtable's client_options_test.cc should always have been using the MakeBackgroundThreadsFactory(opts) method instead of checking the option.

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[google-cloud-cpp-bot]

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[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7354 (1eb2899) into main (c541c55) will increase coverage by 0.01%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##             main    #7354      +/-   ##
==========================================
+ Coverage   93.64%   93.65%   +0.01%     
==========================================
  Files        1349     1349              
  Lines      117050   117064      +14     
==========================================
+ Hits       109607   109635      +28     
+ Misses       7443     7429      -14     

Impacted Files	Coverage Δ
.../bigtable/tests/instance_admin_integration_test.cc	3.76% <0.00%> (+0.02%)	⬆️
google/cloud/grpc_options_test.cc	89.61% <96.15%> (+1.32%)	⬆️
google/cloud/bigtable/benchmarks/benchmark.cc	87.54% <100.00%> (-0.05%)	⬇️
google/cloud/bigtable/client_options.cc	100.00% <100.00%> (ø)
google/cloud/bigtable/client_options_test.cc	99.74% <100.00%> (ø)
google/cloud/bigtable/instance_admin_test.cc	100.00% <100.00%> (ø)
...oud/bigtable/internal/async_longrunning_op_test.cc	100.00% <100.00%> (+0.78%)	⬆️
...ogle/cloud/bigtable/internal/async_poll_op_test.cc	98.91% <100.00%> (+1.04%)	⬆️
...le/internal/async_retry_unary_rpc_and_poll_test.cc	100.00% <100.00%> (+0.74%)	⬆️
google/cloud/bigtable/table_test.cc	100.00% <100.00%> (ø)
... and 10 more

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