feat: make x-goog-user-project work for additional auth clients

[bcoe]

quota_project_id was not working for JWT clients, which are easier to test in our integration environment (which authenticates based on a service account, set in GOOGLE_APPLICATION_CREDENTIALS).

This PR:

1. pulls the logic for populating additional special headers into the AuthClient base class.
2. adds tests to make sure we're getting returned the types of client instances we expect, and that x-goog-user-project is populated for a JWT client.

[codecov]

Codecov Report

Merging #848 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #848      +/-   ##
=========================================
+ Coverage   85.19%   85.2%   +0.01%     
=========================================
  Files          17      17              
  Lines         959     960       +1     
  Branches      215     215              
=========================================
+ Hits          817     818       +1     
  Misses         85      85              
  Partials       57      57

Impacted Files	Coverage Δ
src/auth/authclient.ts	100% <100%> (ø)	⬆️
src/auth/jwtclient.ts	88.63% <100%> (ø)	⬆️
src/auth/oauth2client.ts	83.16% <100%> (-0.17%)	⬇️

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