Allow updating mods from alternative source. #591
Description
This is a continuation of #590.
I would like to implement a mechanism for updating Factorio mods from alternative sources. The first thing that comes to mind would of course be GitHub releases. The primary reason for this request is that I tend to apply small patches to some bigger mods that I play. Updating them manually is a tedious process and while I do have automation for rebasing my changes to new releases, I have no way of automatically deploying this mod to my server in a convenient way.
For this reason, I was wondering about your stance of adding alternative update sources for certain mods. Something in the form of an environment variable like UPDATER_MOD_SOURCE_actual-mod-name=https://github.com/USER/actual-mod-patched/releases/latest/download/package.zip which would instruct the updater to fetch this mod in it's update cycle and compare it to the current version, replacing it in case of a mismatch.
While prototyping this idea I also realised that many mods prohibit redistribution. So it would probably also have a UPDATER_HEADER_field=value environment variable, allowing to download from private/hidden repositories via deployment keys. Which would would fine if you can do all your downloads with the same headers.
Is this something you would feel comfortable with? Feel free to suggest alternative solutions or alterations. I would take care of implementing this feature, if you are happy with the design. What do you think? Please let me know :)