fix: support variables in auth urls

[pseudo-su]

Description

Currently when using variables in your server definitions the auth plugin doesn't interpolate the currently selected value but instead leaves url as https://{environment}.mydomain.com/oauth2/token.

Motivation and Context

Fixes #4740

How Has This Been Tested?

npm run dev using a spec that uses variables in it's server and checking the url that gets used when trying to trigger the auth plugin.

Screenshots (if appropriate):

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[pseudo-su]

@shockey @webron I'm not sure how to reach out and request someone to review this so I'm tagging you here both directly as I noticed you have activity on a recently merged PR, hope that's ok.

[tim-lai]

@pseudo-su Merged! Thanks for the contribution!