feat(scim): render SCIM errors in RFC 7644 form

[xlgmokha]

What kind of change does this PR introduce?

Feature. Adds the RFC-7644 error form and returns it from the SCIM endpoints, so failures under /scim/v2 answer in the shape a SCIM client expects.

Extracted from: #2643

What is the current behavior?

Nothing under /scim/v2 produces a SCIM error. The discovery endpoints return a bare 501 with an empty body, an unknown path under /scim/v2 falls to a plaintext 404, and a POST to a discovery endpoint gets a plaintext 405.

What is the new behavior?

Adds the error message form from RFC-7644 and renders it as application/scim+json.
Handlers can now send a SCIM error by returning one.

• GET /scim/v2/Unknown -> 404 in SCIM form
• POST|PUT|PATCH|DELETE on a discovery endpoints -> 405 in SCIM form, with Allow: GET
• The three discovery endpoints still return 501, now with a SCIM body rather than an empty one

Example:

$ curl -i http://localhost:9999/scim/v2/Unknown
HTTP/1.1 404 Not Found
Content-Type: application/scim+json

{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:Error"
  ],
  "detail": "Endpoint or resource does not exist",
  "status": "404"
}

With GOTRUE_EXPERIMENTAL_SCIM_ENABLED unset, the routes still answer with the ordinary 404 (feature_disabled) rather than a SCIM error.

Additional context