Add remote version checking for non-workshop mods

[Armithaig]

Add a URL to manifest.json pointing to a remote JSON along these lines

{
    "Version": "2.0.8",
    "FilesHash": "D314E9E70312356CCA627F790F114631B3242CF5",
    "SourceHash": "8090C07A2C391A390BB6CF4D382D75B34129880E",
    "PatchNotes": "https://gitlab.com/Armithaig/hearthpyre/tags/v208"
}

All fields optional, if you want to only manually define Version yourself, the remote URL can just be the current manifest.json in your git repo.

(Steam’s also hilariously rubbish at updating its workshop mods, forcing users to un/resubscribe to receive updates, this’d surface when & where that’s needed)