Add support for SAML required_groups

What does this MR do?

This MR lets gitlab maintainers to filter SAML account registrations by group membership.

Are there points in the code the reviewer needs to double check?

N/A

Why was this MR needed?

To avoid license count inflation of bystanders and one-off visitors.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #3143 (closed)