feat(spanner): support x-goog-user-project

[coryan]

Add support to override the project used for billing and accounting
purposes.

Part of the work for #8284

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #8316 (76dd3ae) into main (ddb6a23) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #8316   +/-   ##
=======================================
  Coverage   94.96%   94.97%           
=======================================
  Files        1344     1345    +1     
  Lines      119814   119817    +3     
=======================================
+ Hits       113786   113794    +8     
+ Misses       6028     6023    -5     

Impacted Files	Coverage Δ
google/cloud/spanner/internal/defaults.cc	100.00% <100.00%> (ø)
google/cloud/spanner/internal/defaults_test.cc	100.00% <100.00%> (ø)
google/cloud/completion_queue_test.cc	96.95% <0.00%> (-0.20%)	⬇️
google/cloud/testing_util/scoped_environment.h	100.00% <0.00%> (ø)
...le/cloud/storage/internal/curl_download_request.cc	89.04% <0.00%> (+0.35%)	⬆️
google/cloud/storage/internal/curl_handle.cc	73.03% <0.00%> (+0.56%)	⬆️
google/cloud/bigtable/internal/common_client.h	100.00% <0.00%> (+5.97%)	⬆️

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[dbolduc]

Can/should we test this feature in spanner?

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 76dd3aeca550ba1796185b93c6c05c0542372c96

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• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.