feat: Adds text/json credential_source support to IdentityPoolClients

[bojeil-google]

The spec has been extended to support generic formats for IdentityPoolClients (json and text).

  credential_source: {
    // ...
    format?: {
      type: 'text' | 'json';
      subject_token_field_name?: string;
    };
  };

When text format is specified, the file content or the full URL response is used as the subject token.
When json format is specified, the file content or the URL response is parsed as json and the field with key subject_token_field_name is used to retrieve the subject token.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (byoid@b8f14ec). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             byoid    #1059   +/-   ##
========================================
  Coverage         ?   91.70%           
========================================
  Files            ?       21           
  Lines            ?     4145           
  Branches         ?      497           
========================================
  Hits             ?     3801           
  Misses           ?      344           
  Partials         ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b8f14ec...ae57474. Read the comment docs.

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.