Vendor omniauth-google-oauth2 (v0.6.0) gem

What does this MR do and why?

• 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: !96587 (comment 1083074395)

Screenshots or screen recordings

Google OAuth flow working post-gem upgrade:

Screen_Recording_2022-08-30_at_11.15.22_AM

How to set up and validate locally

1. Create an OAuth app with google: https://developers.google.com/identity/protocols/oauth2
2. Add those credentials to config/gitlab.yml and add google_oauth2 as the allow_single_sign_on option.
3. Restart gdk and visit http://127.0.0.1:3000/users/sign_in and click the "Sign in with Google" button

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.