Add users memberships endpoints for admins

What does this MR do?

It adds a new endpoint to request a list of project or group memberships for a user. This was requested in #20532 (closed).

It takes a variable (type) which can scope it to specific membership types.

GET user/:id/memberships

[
  {
    "source_id": "1111",
    "source_name": "EXAMPLE PROJECT",
    "source_type": "Project"
    "access_level": "PROJECT ACCESS LEVEL"
  },
  {
    "source_id": "123",
    "source_name": "EXAMPLE Group",
    "source_type": "Namespace"
    "access_level": "GROUP ACCESS LEVEL"
  }
[

user/:id/memberships?type=Project would only return project memberships and user/:id/memberships?type=Namespace would return only group memberships.

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

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