Vendor omniauth-google-oauth2 gem

* This is to pave the way for upgrade to OmniAuth 2.0
* OmniAuth v2 supported by 1.0.0+ of omniauth-google-oauth2, but those versions
  also depend on `'oauth2', '~> 1.1'`, which is incompatible with the version
  of oauth2 currently used by GitLab.
* For now, we are vendoring the current version of this gem and will
  upgrade the vendored version in concert with upgrading to OmniAuth 2.0
  because using an updated version of this gem with OmniAuth v1 will
  break relative URL installations of GitLab, see: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/96587#note_1083074395