feat: default self-signed JWTs

[bcoe]

Start using self signed JWTs for client libraries when:

• A service account is being used
• A scope has not been set by the user
• A target_audience has not been set by the user
• An api_endpoint has not been set by the user

This will require upstream work in the client libraries to begin populating defaultScopes rather than scopes.

[codecov]

Codecov Report

Merging #1054 into master will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1054      +/-   ##
==========================================
+ Coverage   91.52%   91.70%   +0.17%     
==========================================
  Files          21       21              
  Lines        4094     4145      +51     
  Branches      488      497       +9     
==========================================
+ Hits         3747     3801      +54     
+ Misses        347      344       -3     

Impacted Files	Coverage Δ
src/auth/googleauth.ts	97.43% <100.00%> (+0.03%)	⬆️
src/auth/jwtaccess.ts	98.57% <100.00%> (+0.21%)	⬆️
src/auth/jwtclient.ts	97.04% <100.00%> (+0.95%)	⬆️

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[bshaffer]

This looks great!

[bcoe]

I believe this is ready for review, when it comes time to integrate, the plan is that:

• gax-nodejs will use the kDefaultScopes symbol to provide a default set of scopes. This allows us to differentiate between user-defined and gax-defined scopes, while discouraging users from setting the key themselves (as you need to go out of your way to do so.).
• gax-nodejs will populate scopes rather than kDefaultScopes if an alternate servicePath has been set.

Note: there's an ongoing discussion about simpler audience strings (this feature will be implemented in a future PR.).

Update: I ended up simplifying the implementation and going back to a string, based on Alex's feedback.

[bshaffer]

First pass. Looks great! Mostly just questions.

[bshaffer]

Should || this.defaultScopes be added at this line as well?

[bcoe]

Should || this.defaultScopes be added at this line as well?

I had missed setting defaultScopes in a couple places -- I'm hoping I caught them all now.