Add API endpoint to get single SAML/SCIM identity

What does this MR do and why?

Adds an API endpoint to allow group owners to get a single SAML/SCIM identity.

Fixes SaaS: API to search for group members by `exter... (#324247 - closed)

Response

SAML

{
  "extern_uid": "GroupIdentity",
  "user_id": 1
}

SCIM

{
  "extern_uid": "GroupIdentity",
  "user_id": 1,
  "active": false
}

How to set up and validate locally

1. Add a SAML and SCIM identity to a user
2. Send a request to the new endpoints
   • http://127.0.0.1:3000/api/v4/groups/:id/saml/:uid
   • http://127.0.0.1:3000/api/v4/groups/:id/scim/:uid

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.