feat: Enterprise SCIM resources (users and groups)

[vmvarela]

Status

Implementation complete. Tracking upstream PR until merged.

• Branch: enterprise-scim
• Upstream PR: [FEAT] Add Enterprise SCIM data sources (groups/users) integrations/terraform-provider-github#3012
• Review status: DRAFT (no reviewers yet)

Description

Implements Terraform resources to manage SCIM in GitHub Enterprise:

• github_enterprise_scim_user
• github_enterprise_scim_group

Acceptance Criteria

• resourceGithubEnterpriseScimUser() with Create/Read/Update/Delete
• resourceGithubEnterpriseScimGroup() with Create/Read/Update/Delete
• Meta schema aligned with SCIMEnterpriseMeta: resource_type, created, last_modified, location
• 404 handled gracefully in Delete
• SetId called after successful API call
• Acceptance tests using t.Run() sub-tests
• Full documentation
• Registered in provider.go and website/github.erb
• Proactive fixes applied (consistency with other PRs)
• PR marked Ready for Review
• Upstream PR approved and merged

Technical Notes

• API: REST SCIM enterprise endpoints
• Import via enterprise_slug:scim_id