Feature Request: use go-getter/v2 for filedownloads

[mildwonkey]

Using go-getter as the implementation of network.Fetch allows downloading local and network files, as well as a number of other network protocols. It also handles credentials for things like git and s3 downloads.

Things to consider: go-getter will download zipped archives and directories, not just files (though we can use GetFile vs. GetAny anywhere we want/need to control that - especially for the initial implementation let's not change the behavior for any pre-existing fetching, but I want this in the files domain), so code that assumes there's a 1-1 relationship between the filepath and the files downloaded likely needs to walk the downloaded filepath to get the list of files in the directory.