Add LDAP scaffolding to group members Vue app [RUN AS-IF-FOSS]

What does this MR do?

Part of &4233 (closed) and #238453 (closed)

I am working on converting the group members page from HAML to Vue. This MR adds the scaffolding needed to implement overriding LDAP roles for group members.

This MR does the following:

• Passes the LDAP override endpoint from HAML to the Vuex store
• Adds the is_overridden property to members
• Moves the can_override property to EE as it should have been in EE in the first place

Currently this is behind the vue_group_members_list feature flag. To enable:

1. bin/rails console
2. Feature.enable(:vue_group_members_list)

Screenshots

No visual changes

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Not needed, behind a feature flag
• [-] 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