Bypass signup domain validation for external users

What does this MR do?

Bypass signup domain validation when adding an external user

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

Why was this MR needed?

From #25279 (closed)

When a company has a hosted GitLab, it makes sense to Whitelist the company domain to ensure that only users (in this case coming through google oauth) from the company are able to gain access. However, this also blocks an admin when creating a new external user.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• #25279 (closed)