Allow users to determine a custom role as the default role when being first provisioned in SAML settings

Currently, we allow users to select a default role when being first provisioned in SAML settings https://docs.gitlab.com/ee/user/group/saml_sso/#configure-gitlab (4.)

The request here is that a custom role should also be visible in that list to be used as a default role e.g everyone in the org gets a Guest+1