[Feature]: Limit user login methods

[infinitetrooper]

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In the scenario of partners managing clients' instances, the partner admins usually don't have SSO login that client users possess. They will need to manage these instances through form login but, this opens up the form sign-up method also to client users.

This is a cause for confusion in 2 aspects

1. If sign-up is allowed in the instance, this setting will allow anyone to sign up using the form method and is a security concern.
2. If sign-up is disabled, the invited users may also choose to log in through the form method defeating the purpose of SSO.

We need to enable the admins to limit sign-ups to certain auth methods to allow for more granular instance management in this scenario.