fix: respect GOOGLE_CLOUD_QUOTA_PROJECT

[dbolduc]

https://google.aip.dev/auth/4110 says we should have used the env var GOOGLE_CLOUD_QUOTA_PROJECT instead of making up our own GOOGLE_CLOUD_CPP_USER_PROJECT.

So we will respect both.

Note that this is separate from our QuotaUserOption which sets the x-goog-quota-user header. Confusing stuff.

I do not think we need to publicize the new form of the env var.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.62%. Comparing base (da56bf9) to head (7bb8cd0).
Report is 109 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14684   +/-   ##
=======================================
  Coverage   93.61%   93.62%           
=======================================
  Files        2320     2320           
  Lines      207188   207197    +9     
=======================================
+ Hits       193966   193981   +15     
+ Misses      13222    13216    -6     

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[scotthart]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dbolduc)