API for transfer progress stats

There has been a fair amount of demand for a way to check the 'transfer progress' of a given file, similarly to how torrent clients show.

This is not currently a very easy thing to track, but i think that we can abuse context values and the commands diagnostics api (`ipfs diag cmds`) codebase to get something working here.

We first should design what we want this api to look like, and what format the information it provides should be in. Then we can go ahead and try to implement it.