Implement mfa/associate endpoint

[czf]

Checklist

• [X] I have looked into the Readme 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

The authentication client doesn't support mfa/associate

Describe the ideal solution

The authentication client t supports mfa/associate

Alternatives and current workarounds

No response

Additional context

No response