Add cross-instance messenger pubsub setup

[rijkvanzanten]

Description

Adds a Messenger system to Directus, which allows for cross-instance communication. This is currently used to fix #13650, but can in the future also be used for WebSockets, syncing CRON jobs, and other load balanced operations.

Every instance is both a publisher and a subscriber, which allows any instance to trigger any change on other instances.

When MESSENGER_STORE isn't configured to redis, it'll default to a no-op fallback (there's no need for a messenger setup like this if you're running Directus on a single instance)

Fixes #13650

Request to instance w/ port 8055 triggers RELOAD on only 8056 and vice versa:

CleanShot.2022-05-31.at.18.34.45.mp4

Type of Change

• Bugfix
• New Feature
• Refactor / codestyle updates
• Other, please describe:

Requirements Checklist

• New / updated tests are included
• All tests are passing locally
• Performed a self-review of the submitted code

If adding a new feature:

• Documentation was added/updated

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Left to-do:

• Docs