Feature Request: Allow specifying a new local path for an already uploaded file

This may overlap with #2256 to some degree but that one is about multiple things while this one here is about one very specific issue.

If the local path for a file changes (because it's moved to a different disk/folder or renamed in some other form), it would be great if we could tell siad about that and it could use the new location for maintaining redundancy.

This should be address with the new file format #2204