Add SCIM identities to User API response

What does this MR do and why?

In alignment with the admin area when viewing a user profile within the Identities tab, this MR adds SCIM identities to the Users API response.

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

Screenshots or screen recordings

How to set up and validate locally

• Add a SCIM identity to a user
• Send a request to the Users API endpoint

MR acceptance checklist

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