SAML Group Links permission policies

What does this MR do?

Adds group policies for SAML group links. Also cleans up existing group SAML policies and adds tests for everything.

Part of #267019 (closed). The next step will be to introduce controllers and views for managing group links. That is where these new and adjusted policies will be used.

I extracted this out of another MR since it was getting a bit long - !45080 (merged).

I didn't add a changelog since this is all backend changes that won't affect the user.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team