feat(kms): support the EKM service

[coryan]

This is a new gRPC service in the same googleapis directory (google/cloud/kms/v1) so it goes into the same library.

Fixes #8483.

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[google-cloud-cpp-bot]

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Codecov Report

Merging #8530 (8dc851a) into main (70e0a81) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #8530      +/-   ##
==========================================
- Coverage   93.92%   93.92%   -0.01%     
==========================================
  Files        1453     1453              
  Lines      125889   125889              
==========================================
- Hits       118242   118241       -1     
- Misses       7647     7648       +1     

Impacted Files	Coverage Δ
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (-0.57%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (ø)

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[dbolduc]

1. I think we want a note somewhere in the CHANGELOG that this service has been added. Like we did in cleanup: touch up CHANGELOG #8422 for bigquery
2. Can you add the new environment variable here:

   google-cloud-cpp/google/cloud/kms/doc/main.dox

   Lines 44 to 46 in 70e0a81

   	- `GOOGLE_CLOUD_CPP_KEY_MANAGEMENT_SERVICE_ENDPOINT=...` changes the default endpoint
   	(cloudkms.googleapis.com) used by `KeyManagementServiceConnection`.

This makes me think we may want/need a section in our howto doc for "adding new services to GA libraries". Even if it's just a two item checklist. What do you think?

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