chore(scim): deliver SCIM responses through a protocol helper

[xlgmokha]

Extract SendJSONAs from SendJSON so callers can specify a Content-Type, and add a protocol package that wraps it with the SCIM media type.

Extracted from #2643

What kind of change does this PR introduce?

Refactor. Extracts a scim/protocol package that writes responses using the SCIM media type, so the discovery endpoints in the follow-up PRs have one place to send from.

What is the current behavior?

shared.SendJSON hardcodes application/json, so it is not usable for SCIM.
scim.Server.notImplemented writes its response by hand instead, setting Content-Type from a local mediaType const and calling WriteHeader(501) directly.

What is the new behavior?

• shared.SendJSONAs(w, status, contentType, obj) takes the Content-Type; SendJSON delegates to it with application/json.
• A new internal/api/scim/protocol package holds MediaType (application/scim+json) and Send.
• notImplemented goes through protocol.Send, and the local mediaType const is gone.

Additional context