feature(oauth2): allow passing additional form params to token endpoint

[notpushkin]

Description

This PR adds a config variable allowing users to add arbitrary fields to the form body sent to the token endpoint.

Motivation and Context

Some OAuth2 providers, like Logto, accept additional parameters in the body of the token endpoint. For example, if you pass resource=https://api.example.net, it would issue access_token for that particular API.

I'm not yet sure if this is a valid OAuth2 implementation detail, but it should make the configuration more versatile and allow working with more identity providers.

How Has This Been Tested?

Not yet :') I'll be grateful for pointers to how to cover it with unit / integrations tests!

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.