Implement `POST /Groups` endpoint

Description

Implement SCIM Groups provisioning endpoint (POST /Groups) for self-managed instances that enables identity providers to associate their SCIM group IDs with existing SAML group links in GitLab. This allows synchronization between IdP groups and GitLab groups via SAML group links.

Acceptance Criteria

• Endpoint accepts SCIM-formatted POST requests with displayName (SAML group name) and externalId (SCIM group ID)
• Endpoint requires a valid SCIM access token
• Feature is only available when self_managed_scim_group_sync feature flag is enabled
• Finding existing SAML group links matches on the provided SAML group name
• Updates matching SAML group links with the provided SCIM group ID
• Returns 201 status with SCIM-formatted group details on success
• Returns appropriate SCIM error responses when SAML group not found
• Only available for self-managed instances (not on GitLab.com)
• Only available when instance SAML is configured
• Only available with proper license that includes instance-level SCIM

Example Request

POST /api/scim/v2/application/Groups
Accept: application/scim+json
Content-Type: application/scim+json
Authorization: Bearer <token>

{
  "displayName": "engineering",
  "externalId": "scim-group-123"
}

Example Response

{
  "schemas": ["urn:ietf:params:scim:schemas:core:2.0:Group"],
  "id": "engineering",
  "displayName": "engineering",
  "externalId": "scim-group-123"
}