Restrict login to SSO only

**Is your feature request related to a problem? Please describe.**
- Users should be forced to use MFA when logging in to the admin (core) dashboard (not via username/password field)
- Solution should not make it more complicated for the user to use the system if SSO is available

**Describe the solution you'd like**
- Option to restrict login via SSO only
-> Avoids the need to configure MFA inside Defguard
-> Single point of configuration (here: inside SSO only)

**Describe alternatives you've considered**
None, except telling future users to enable MFA.

**Additional context**
None