Add skin `index.json` to skin databases to support listing all skins in client

[Robyt3]

There should be a way for skin databases to provide an index of all skins they offer as a json file.

This is necessary so the client could fetch this index and show a list of all available downloaded skins directly in the menu (with lazy loading and unloading, which is work-in-progress).

Currently, one could parse the directory listing of https://skins.ddnet.org/skin/ but a standardized way like adding a file index.json to every skin download URL would be much preferred.

I suggest the following extensible format:

{
	"skins": [
		{
			"name": "topaz"
		}
	]
}

Which could be extended by other attributes which may or may not be shown in the client or be used for grouping:

{
	"skins": [
		{
			"name": "topaz",
			"creator": "Pipou",
			"pack": "Gems",
			"license": "CC BY-NC-SA"
		}
	]
}

[Jupeyy]

There is an index:
https://skins.ddnet.org/skin/skins.json

Currently generated by the upload php script.
It's e.g. missing a sha256 or similar.

Could also switch to ddnet-rs' implementation for assets (more generalized):
https://github.com/ddnet/ddnet-rs/tree/fa26d967b00aecce0dfd1c267c442c986fd0a104/src/assets-server

It supports skins by name.png aswell as by name_blake3hash.png (could easily be extended to allow sha256 too)
It would require some kind of patch to allow community skins.

Additionally it supports a meta.json which can fill authors, liceneses, tags etc. independent of the index too keep it small.

[Robyt3]

Oh, nice. But could we also make this index work when changing the skin download URL in the client? For example there is currently no index at https://skins.ddnet.org/skin/community/skins.json so you'd always have to fetch the index from https://skins.ddnet.org/skin/skins.json. If the skin download URL is changed to a custom server then I think we should fetch the index from that server to know which skins it offers.

[Jupeyy]

Oh, nice. But could we also make this index work when changing the skin download URL in the client? For example there is currently no index at https://skins.ddnet.org/skin/community/skins.json so you'd always have to fetch the index from https://skins.ddnet.org/skin/skins.json. If the skin download URL is changed to a custom server then I think we should fetch the index from that server to know which skins it offers.

Yeah having one index per skin category would simplify the server side code also for the upload process.
Would make the discord bot & website upload little more complex when switching a skin from e.g. normal to community.

Only advantage of the current approach is for the website skin list, since it doesn't need to fetch multiple indices.