feat(scim): implement /scim/v2/ServiceProviderConfig

[xlgmokha]

What kind of change does this PR introduce?

Feature. GET /scim/v2/ServiceProviderConfig returns a real RFC-7643 document.

Extracted from #2643

What is the current behavior?

All three discovery endpoints return 501 in the RFC-7644 error form.
Nothing describes what this server supports, so a SCIM client has no way to discover its capabilities.

What is the new behavior?

A new scim/core package holds the SCIM 2.0 core schema types from RFC-7643.

The /ServiceProviderConfig endpoint reports every attribute RFC-7643 marks as REQUIRED. Each capability is set to supported: false.

authenticationSchemes advertises the OAuth bearer token scheme. A later PR will enforce it.

$ curl -s 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"
  }
}

Additional context