feat: Add audience support during authorization

[ruhulio]

Description

Adds support for passing the audience to the upstream provider if it was specified on the token verifier. This is necessary when using a provider like Auth0 that requires audience scoping for the tokens.

This was manually tested via MCP Inspector against an Auth0 app for the positive case, and a GitHub app for the negative case.

This can be considered a smaller peer change to the recent PKCE forwarding changes (#1733).

Contributors Checklist

• My change is related to issue #(issue number)
• I have followed the repository's development workflow
• I have tested my changes manually and by adding relevant tests
• I have performed all required documentation updates

Review Checklist

• I have self-reviewed my changes
• My Pull Request is ready for review

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[dmikusa]

I hit this issue with Auth0 too, so +1 for this. It'd be nice if it just worked automatically.

FYI, if anyone else hits this. I worked around it by setting .../authorize?audience=<my-audience> as my upstream_authorization_endpoint value. It was smart enough to retain that audience query param.