[IAMRISK-3539] Add new endpoint for signup challenge

[TSLarson]

Changes

Adds new signup challenge endpoint endpoint

References

Needs this PR merged in order to work: https://github.com/atko-cic/auth0-server/pull/13407 JIRA: https://auth0team.atlassian.net/browse/IAMRISK-3539

Testing

I tested this locally with vivaldi. I ran my corresponding server PR locally. I linked this PR in lock and tested that the new signup endpoint is returning current default settings for captcha.

• [x] This change adds unit test coverage
• [ ] This change adds integration test coverage

Checklist

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