Merge branch 'pb/509427/scim-self-managed-get-group' into 'master'

Implement `GET /Groups/:id` SCIM endpoint for self-managed

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/181577



Merged-by: Roy Zwambag <rzwambag@gitlab.com>
Approved-by: Omar Qunsul <oqunsul@gitlab.com>
Approved-by: Drew Blessing <drew@gitlab.com>
Reviewed-by: Omar Qunsul <oqunsul@gitlab.com>
Reviewed-by: Paulo Barros <pbarros@gitlab.com>
Reviewed-by: Mohamed Hamda <mhamda@gitlab.com>
Co-authored-by: Paulo Barros <pbarros@gitlab.com>