feat: add self signed jwt support

[arithmetic1728]

This PR adds self signed jwt support (https://google.aip.dev/auth/4111). Googlers see this go/yoshi-self-signed-jwt for more details. If user uses service account credentials, and doesn't provide custom endpoint/scopes/audience, then self signed jwt should be used. Since custom endpoint is now supported (cl/356594013) and users cannot provide default audience for ServiceAccountCredentials, we automatically use self signed jwt if scopes are not provided by the users.

Details

1. Scopes.
   We add a new defaultScopes (for client lib) to GoogleCredentials to distinguish with the existing scopes (for users), and updated the subclasses.

• For AppEngineCredentials, ComputeEngineCredentials classes, defaultScopes will used if scopes is not provided, there are no other changes.
• For ServiceAccountCredentials, if scopes is not provided, then a ServiceAccountJWTAccessCredentials instance will be created inside ServiceAccountCredentials, and will be used for self signed jwt.
• For other type of credentials where scopes are not used, defaultScopes has no effect.

2. Simplified self signed jwt audience. Googlers see go/simpler-JWT-token-audience
   When self signed jwt is used for service account credentials, in getRequestMetadata(uri), we replace the uri with its http(s)://{host_name}/ prefix. For instance, http client normally passes uri like https://compute.googleapis.com/compute/v1/projects/, but the correct audience should be https://compute.googleapis.com/.

Follow up PRs

It will be trivial changes to gax-java and client libs to integrate this feature.

This PR shows how gax-java integrates the feature. gax-java just needs to pass defaultScopes to the auth lib.

This PR shows how GAPIC client integrates the feature. GAPIC client just needs to pass the default scopes via setDefaultScopes instead of setScopesToApply.

[codecov]

Codecov Report

Merging #572 (8af2eaa) into master (497d4e7) will increase coverage by 0.12%.
The diff coverage is 86.15%.

@@             Coverage Diff              @@
##             master     #572      +/-   ##
============================================
+ Coverage     83.32%   83.45%   +0.12%     
- Complexity      571      594      +23     
============================================
  Files            41       41              
  Lines          2645     2695      +50     
  Branches        274      286      +12     
============================================
+ Hits           2204     2249      +45     
- Misses          301      303       +2     
- Partials        140      143       +3     

Impacted Files	Coverage Δ	Complexity Δ
...java/com/google/auth/oauth2/GoogleCredentials.java	61.70% <0.00%> (-1.35%)	11.00 <0.00> (ø)
...google/auth/oauth2/DefaultCredentialsProvider.java	81.39% <50.00%> (+0.92%)	34.00 <0.00> (+1.00)
...uth/oauth2/ServiceAccountJwtAccessCredentials.java	75.30% <71.42%> (-0.34%)	42.00 <6.00> (+2.00)	⬇️
.../google/auth/oauth2/ServiceAccountCredentials.java	83.44% <88.88%> (+0.50%)	64.00 <17.00> (+12.00)
...a/com/google/auth/oauth2/AppEngineCredentials.java	86.36% <91.66%> (+1.61%)	19.00 <8.00> (+4.00)
...m/google/auth/oauth2/ComputeEngineCredentials.java	84.86% <100.00%> (+0.97%)	39.00 <7.00> (+4.00)

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[arithmetic1728]

@bshaffer Hi Brent, could you take a look?

[arithmetic1728]

Don't merge until cl/356594013 is fully rolled out.

[arithmetic1728]

cl/356594013 is fully rolled out. Tested the change with pubsub regional endpoint and it worked fine.

[suztomo]

@arithmetic1728 @bshaffer It seems that this commit (the only change between 0.24.1 and 0.25.0) broke the Java sample projects. GoogleCloudPlatform/java-docs-samples#4903 Would you look into the failures?

I've invited both of you to "Yoshi Java" chat room.

[elefeint]

It also broke Spring Cloud GCP by way of bigquery, config, storage and vision: https://github.com/GoogleCloudPlatform/spring-cloud-gcp/actions/runs/664693361

[ejona86]

@arithmetic1728, when merging PRs, could you please use helpful commit messages? You wrote so much good content for this PR, but then it is mostly just garbage on the commit itself: efe103a

[arithmetic1728]

@ejona86 Yep, I will edit the commit message in the future when merging PR. Thank you for pointing it out.