Server setup:

resources_url = https://someplace.io/skins/textures

Client operation:

/skin set player name

Result

Look for (in order)

1. https://someplace.io/skins/textures/name.png
2. MC skins
3. ely.by

Alternatives

/skin set name : not using either 'player', 'upload' or 'url' means you are using the default skin server (as configured on the server config)
/skin set url name : if url doesn't contain 'http://' or 'https://' consider it needs to be 'completed' with the server-configured url prefix

Shouldn't be too hard actually.
It would probably come out as "https://myskinsite.com/.png

RN I'm in a middle of refactor (threading), so this might need to wait.
Feel free to PR it if you want ;)

It's been a while and I'm (finally) implementing this ...

How should slim/default skins be handled?

Perhaps a "custom" category should be better (/skin set custom <model> <name>) ...

I see you have worked on connecting external skin sources, but I can't figure out how to connect a custom skin server

use 2.0.0; set the config option for custom skin server url to wanted protocol (can be also file://) and run /skin set custom <model> <skin_filename>

Can you publish build for 1.18.2?

Sorry there've been too many changes to the code :/, I'd rather spent time on other stuff than backporting.