Evaluate options for sync backend implementation

[juliusknorr]

• Requires #7
• Blocks #3

We shoudl evaluate pros and cons of implementing a custom sync backend

• Option 1: Plain PHP polling based with notify push extension (like in text)
• Option 2: Separate service like https://github.com/excalidraw/excalidraw-room / https://github.com/jitsi/excalidraw-backend

Further references

• Excalidraw seems to use socket.io with also supports longpolling (https://socket.io/docs/v3/how-it-works/) which Robin also built around in https://github.com/nextcloud/documentserver_community
• Consider horizontal scaling (based on file id)
• https://github.com/excalidraw/excalidraw/issues/2213#issuecomment-704983925