Add OpenID Connect OmniAuth support

What does this merge request do and why?

This commit adds support for enabling OpenID Connect as an OmniAuth provider (https://docs.gitlab.com/ee/administration/auth/oidc.html).

Since there are so many configuration options, just leave the complexity to the args hash.

How to set up and validate locally

1. Choose your favorite provider (e.g. Google): https://docs.gitlab.com/ee/administration/auth/oidc.html
2. Configure it (see gdk.example.yml as an enable).
3. Attempt to login/connect your account with that provider

Sample config:

omniauth:
  openid_connect:
    enabled: true
    args:
      name: "openid_connect"
      scope: ["openid", "profile", "email"]
      response_type: "code"
      issuer: "https://accounts.google.com"
      client_auth_method: "query"
      discovery: true
      uid_field: "preferred_username"
      pkce: true
      client_options:
        identifier: "<YOUR OAUTH2 CLIENT ID>"
        secret: "<YOUR OAUTH2 SECRET>"
        redirect_uri: "https://localhost:3443/users/auth/openid_connect/callback"

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.