feat: SCIM discovery endpoints

[xlgmokha]

What kind of change does this PR introduce?

This change implements the SCIM discovery/metadata endpoints behind the experimental feature flag from #2639:

• GET /scim/v2/ServiceProviderConfig
• GET /scim/v2/ResourceTypes
• GET /scim/v2/Schemas

These replace the 501 Not Implemented stubs with spec-compliant JSON responses.

https://linear.app/supabase/issue/AUTH-1362/scim-discovery-endpoints

What is the current behavior?

With #2639 alone, the three routes above mount when the flag is on, but each just returns a bare 501 with no body.

What is the new behavior?

Each endpoint now returns real RFC 7643-compliant JSON responses:

• ServiceProviderConfig reports this server's actual current capabilities. (authenticationSchemes user bearer-token scheme that a later PR will enforce.)
• ResourceTypes and Schemas return an empty Resources: [] list, since no resource types (Users, Groups) exist yet.

Nothing here is reachable unless GOTRUE_EXPERIMENTAL_SCIM_ENABLED is set.

GET /scim/v2/ServiceProviderConfig

モ curl http://localhost:9999/scim/v2/ServiceProviderConfig | jq '.'
{
  "schemas": [
    "urn:ietf:params:scim:schemas:core:2.0:ServiceProviderConfig"
  ],
  "patch": {
    "supported": false
  },
  "bulk": {
    "supported": false,
    "maxOperations": 0,
    "maxPayloadSize": 0
  },
  "filter": {
    "supported": false,
    "maxResults": 0
  },
  "changePassword": {
    "supported": false
  },
  "sort": {
    "supported": false
  },
  "etag": {
    "supported": false
  },
  "authenticationSchemes": [
    {
      "type": "oauthbearertoken",
      "name": "OAuth Bearer Token",
      "description": "Authentication scheme using the OAuth Bearer Token Standard",
      "specUri": "http://www.rfc-editor.org/info/rfc6750",
      "primary": true
    }
  ],
  "meta": {
    "resourceType": "ServiceProviderConfig",
    "location": "http://localhost:9999/scim/v2/ServiceProviderConfig"
  }
}

GET /scim/v2/ResourceTypes

モ curl http://localhost:9999/scim/v2/ResourceTypes | jq '.'
{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:ListResponse"
  ],
  "totalResults": 0,
  "startIndex": 1,
  "itemsPerPage": 0,
  "Resources": []
}

GET /scim/v2/Schemas

モ curl http://localhost:9999/scim/v2/Schemas | jq '.'
{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:ListResponse"
  ],
  "totalResults": 0,
  "startIndex": 1,
  "itemsPerPage": 0,
  "Resources": []
}

404 Error:

モ curl http://localhost:9999/scim/v2/ | jq '.'
{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:Error"
  ],
  "detail": "Endpoint or resource does not exist",
  "status": "404"
}

405 Error:

モ curl -XPOST http://localhost:9999/scim/v2/ResourceTypes | jq '.'
{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:Error"
  ],
  "detail": "The request method is not supported by this endpoint",
  "status": "405"
}

Additional context