Implement OpenID Connect identity provider

What does this MR do?

This implements an OpenID Connect 1.0 identity provider on top of the Doorkeeper OAuth framework (which is already used in Gitlab to manage API access), using the doorkeeper-openid_connect gem.

As per the discussion with @timothyandrew, we decided to only use the openid scope for now, and make additional user attributes available there as well (the spec recommends putting them into the additional profile and email scopes). See doc/integration/openid_connect_provider.md for more details.

This development is sponsored by @siemens (/cc @bufferoverflow)

Are there points in the code the reviewer needs to double check?

• Pending Omnibus MR: https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1222

Why was this MR needed?

Adding OpenID Connect IdP functionality would simplify the usage of GitLab as identity provider by supporting a standard protocol that is already widely adopted and standardized, and enables simple cross vendor scenarios without implementing custom glue code per provider on client side.

Screenshots (if relevant)

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Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
  • the extension only becomes active for OpenID Connect requests (OAuth request with openid scope), so normal Doorkeeper usage shouldn't be noticably affected
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/22323

/cc authentication oauth