SAML group link controller

What does this MR do?

Allows groups to create/manage SAML group links. This covers the new routes, controllers, and views. Related to #267019 (closed). The setup experience will be similar to LDAP group mapping.

Feature Flag: This also creates a development feature flag :saml_group_links scoped to group since the feature would currently not do anything beyond creating the link - no actual user membership sync will occur until #267020 (closed) is implemented.

Documentation: I did not include documentation here since the feature is not yet usable. I think it is most appropriate to include documentation as part of #267019 (closed).

Changelog: I did not include a Changelog since this is behind a disabled feature flag.

Screenshots

Navigation

Page

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