feat: download from SP returns uploaded file rather than CAR

[SgtPooki]

pulled from figma comment

As a result, in the browser we'll need to fetch https://$spdomain/piece/$filecoinPieceCid, which gives us the CAR that was uploaded. The CAR then needs to be process by JS car library and JS triggers a file save to the OS for "filecoin-corgi.png".

We should be able to do stream the download from the SP into a car reader, which then transforms parsed content for the file, and then stream the actual content bytes with proper filetype are streamed to the users machine.

There will likely be some intermediary memory usage growth while parsing the car and then walking the dag for cars containing multiple files. (we could use indexeddb blockstore to cache this for this usecase?)

[rvagg]

directories are going to be a problem for this if we're allowing upload of directories -- what are we allowing them to download?

but the trustless http gateway gives us all the tools we'd need to explore and download and render what we need and we shuldn't even need to cache anything, we don't use the /piece/ endpoint, we use the /ipfs/ endpoint to selectively download the content we need and put dups=y back in so we don't need to hold on to blocks, the next block in the CAR is always the one we need to render the selected content

the biggest problem here is the work involved and figuring out how what we're rendering, where it's being rendered to, is it just a download button that gives you a stream of rendered content? what to do if it's a directory? is it a multi-download then?

@bajtos was playing with this recently and had a flow working that demonstrates it: https://github.com/bajtos/poc-car-stream-to-raw-stream/blob/main/src/index.ts - note the URL /ipfs/${cid}${subpath}?format=car&dag-scope=entity&car-dups=y - dups=y makes sure that the next block is always the block we want when we traverse it in standard order, so we can just treat the CAR block stream as a block store, every get() should return us the block we want (and if not, there's something badly wrong).