Geo: Add setting for ActionCable allowed origins

What does this merge request do and why?

This add a new setting to enable admins to setup allowed origins for websocket requests through ActionCable in order to ensure communication between secondary and primary Geo sites via websockets works. This fixes a bug where Duo is not working from secondary sites.

This setting should be added at the very end of an omnibus Geo installation, after the secondary sites have been added with external and internal URLs (single node doc).

Related issues

• bug issue: Websocket requests (impacting Classic Duo Chat)... (gitlab#579074)
• Rails MR: Geo: Add setting for ActionCable allowed origins (gitlab!213206)
• Charts MR: gitlab-org/charts/gitlab!4644
• Omnibus MR: omnibus-gitlab!8910

How to set up and validate locally

TBD

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 MR references an issue describing the change.
• This change is backward compatible. If not, include steps to communicate to users.
• Tests added for new functionality. If not, raise an issue to follow-up.
• Observability added/updated (logging, metrics, tracing).
• Documentation added/updated.
• Announcement added for notable changes.
• gdk doctor test added.