feat: allow ServiceAccountJwtAccessCredentials to sign scopes

[bshaffer]

TODO

• Merge and tag feat: allow ServiceAccountJwtAccessCredentials to sign scopes google-auth-library-php#341
• Update minimum google/auth version in composer.json to the new tag (v1.16.0)
• Add 'useJwtAccessWithScopes' => false to gapic-generator-php for DIREGAPIC APIs (done in feat: implement jwt access option gapic-generator-php#309)
• Generate a new Gapic Compute client with the changes in feat: implement jwt access option gapic-generator-php#309

Note: These steps are now optional because Compute does not need the exclusion for Self-Signed JWTs with Scopes

• Merge chore: manually re-generate Compute for JWT changes to gapic generator google-cloud-php#4199
• Tag a new version of google/cloud-compute
• Merge this PR
• Tag a new version of this library (google/gax)
• Update Gapic Compute client requires the latest tag of google/gax