Provide a way to override WebAuth config for custom ULP

[tomwyr]

Checklist

• [X] I have looked into the Readme, Examples, and FAQ and have not found a suitable solution or answer.
• [X] I have looked into the API documentation and have not found a suitable solution or answer.
• [X] I have searched the issues and have not found a suitable solution or answer.
• [X] I have searched the Auth0 Community forums and have not found a suitable solution or answer.
• [X] I agree to the terms within the Auth0 Code of Conduct.

Describe the problem you'd like to have solved

Currently, it doesn't seem to be possible to provide overrides object when configuring Auth0 sdk in Flutter. According to Auth0 docs, it is necessary to override some of those options in order for a customized Universal Login page to work properly.

Describe the ideal solution

Allow specifying a typed or arbitrary overrides object that allows configuring options needed for custom login page.

Alternatives and current workarounds

No response

Additional context

No response

[tomwyr]

The mentioned documentation refers to Auth0.js which is a frontend web library. Flutter uses SPA SDK which doesn't seem to require any additional configuration.