Vendor omniauth-gitlab gem to provide backwards compatibility

This commit vendors the omniauth-gem v4.0.0 to support:

1. OmniAuth v1 and v2. OmniAuth v2 disables GET requests by default
   and defaults to POST. GitLab already has patched v1 to use POST,
   but other dependencies need to be updated:
   https://gitlab.com/gitlab-org/gitlab/-/issues/30073.

2. [`oauth2`](https://github.com/oauth-xx/oauth2) v1.4.9 and up.
   [v1.4.9 fixed relative URL
   handling](https://github.com/oauth-xx/oauth2/pull/469).

   However, this breaks the default GitLab.com configuration and
   existing configurations that use the `/api/v4` suffix in the `site`
   parameter.

   [omniauth-gitlab v4.0.0 fixed the first
   issue](https://github.com/linchus/omniauth-gitlab/pull/22), but the
   second issue requires an admin to update `site` to drop the suffix.

   This fork restores backwards compatibility that was removed in
   omniauth-gitlab v2.0.0.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/361284

Changelog: changed