chore: enable self-mutation on fork PRs and use GithubCredentials API

[mrgrain]

This PR improves the CI automation workflows by using projen's GithubCredentials API and enabling self-mutation on fork PRs.

The existing self-mutation job in the build workflow only worked for PRs from the same repository. Fork PRs would fail the build if projen-generated files were outdated, with no way to automatically fix them. This change introduces a separate self-mutation workflow that triggers on build completion and can apply patches to fork PRs as well.

Additionally, this migrates all workflow token handling to use projen's GithubCredentials API instead of raw secret references. This provides a consistent way to configure authentication across workflows and enables the use of GitHub environments for secret access control.

Key changes:

• Add SelfMutationOnForks projen component that creates a dedicated self-mutation workflow
• Use GithubCredentials.fromPersonalAccessToken() with the automation environment
• Refactor GitHubToken utility from enum to class using GithubCredentials
• Update issue-labeler and pr-labeler to use the new credentials API
• Upgrade projen to 0.99.7 for the GithubCredentials API improvements

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.69%. Comparing base (1bdbaca) to head (f75a897).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1056      +/-   ##
==========================================
- Coverage   87.70%   87.69%   -0.01%     
==========================================
  Files          72       72              
  Lines       10095    10095              
  Branches     1332     1332              
==========================================
- Hits         8854     8853       -1     
- Misses       1216     1217       +1     
  Partials       25       25              

Flag	Coverage Δ
suite.unit	87.69% <ø> (-0.01%)	⬇️

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