Allow gitlab_group_saml_link to use custom roles

When I try to assign a custom role to the access_level field in gitlab_group_saml_link, I receive this error:

│ Error: expected access_level to be one of ["guest" "reporter" "developer" "maintainer" "owner"], got developer-with-terraform-and-cicd-vars

The UI currently supports setting custom roles here; please consider supporting in terraform/tofu.

(Apologies if I missed an existing ticket for this. Still learning gitlab issue-searching tricks.)