the resuming logic is super duplicating and was also somewhat
divergent between the duplicates.

to resolve this there's now a new unified couple of helpers that
specifically deal with establishing context on a file transfer. this
actually maybe should eventually move to KIO proper. it seems to me that
every implementation needs to do more or less the same thing except for
the IO specifics

- shouldResume is called relatively early on and establishes the
context. the context is entirely dependent on the resume configuration
at hand. if KIO::Resume is set we resume in-place, otherwise we may have
an intermediate .part file. if neither is applicable then we straight up
need to overwrite. this effectively hides the details of the
intermediate url switch. the actual transfer logic in smb_dir gets a
context back with the destination configured to where it should write to
(not necessarily where the file will end up at in the end)
- concludeResumeHasError is call...