Groups::OmniauthCallbacksController#group_saml issues long database transactions

On GitLab.com, Groups::OmniauthCallbacksController#group_saml has been identified to issue long database transactions. Based on the sampling of pg_stat_activity, we have seen transaction timings go up to about 1 minute.

This is issue is to track investigation and proposed steps to change the design, so that we reduce the length of individual database transactions related to this worker.

See https://gitlab.com/gitlab-org/gitlab/-/issues/342843 for more statistics.