Add screen_hint to allowed parameters

[tomaszn]

Changes

This PR adds support for new "screen_hint" option, which allows showing the signup page instead of the signin page; fixes #1092.

References

Feature description: https://auth0.com/docs/universal-login/new#signup.
Documentation needs to be updated: https://auth0.com/docs/libraries/auth0js/v9#webauth-authorize-.

Testing

The typical usage looks as follows:

webauth.authorize({screen_hint: 'signup'})

Sign Up tab should be displayed on the Universal Login Page.

• This change adds unit test coverage
• This change adds integration test coverage

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All tests and linters described in the Develop section run without errors

[stevehobbsdev]

Thanks @tomaszn, and thanks for pointing out the discrepancy with the docs!