[FR] Nextcloud integration

### Description

I currently self-host a Nextcloud server for my personal files and documents. It's a pretty convenient option to have services integrate with Nextcloud and use it as a storage bucket instead of relying on a bespoke server that I need to configure and self-host alongside my current solutions.

Integrating Appflowy to use Nextcloud as its storage provider would allow me to utilise my server without any configuration or maintenance that a second server would necessitate, even if it came at the cost of performance or certain features.

### Impact

Appflowy's cloud server seems promising, but in order to integrate it to my raspberry pi server I'll need to set up a new database, and build the server from source as it isn't provided within my distro's package manager (DietPi). It would be a lot easier to just point my apps to my Nextcloud instance and sync it from there.

Also, using it from within nextcloud means I can browse content such as uploaded images.

### Additional Context

I imagine this could be as simple as telling appflowy to act like the nextcloud folders are no different to any other storage location. You could go further and sync files more intelligently with a local copy, such as checking timestamps to overwrite files only if they've been changed recently or the like.