Implement "Restrict group access by domain"

I did implement it in xanzy/go-gitlab: https://github.com/xanzy/go-gitlab/pull/2059

I will probably implement it here where there is a new release.

Proposal:

resource "gitlab_group" "example" {
  name           = "example"
  path           = "example"
  allowed_email_domains_list = [
    "example.com",
    "gitlab.com",
  ]
}

The field is only available on the group update API.

https://docs.gitlab.com/ee/user/group/access_and_permissions.html#restrict-group-access-by-domain