fix: download HAMT-sharded directories

[hacdias]

fixes #1885

License: MIT
Signed-off-by: Henrique Dias [email protected]

[lidel]

LGTM.

Disclaimer: this is the best we could do, given what go-ipfs 0.11 provides:

• object patch is deprecated. It is not aware of HAMT-sharding, nor it will keep block size under 1MB
  • unsafe to use, we have to remove this ASAP
• Tracking issue for UnixFS automatic sharding kubo#8106 made MFS pretty smart: it will autoshard/unshard and keep block sizes in check
  • the downside is that this MFS dance looks a bit awkward: it "pollutes" MFS root with temporary directory, but we prefix it with zzzz so it should not impact GUIs even when Files screen is opened at the time of execution.
  • we could pin the ephemeral CID to ensure its not GC'd after removal from MFS, but in practice it's not worth bothering given this is only for download action in the desktop app
• this only solves block/hamt problem (when object patch mutaterd HAMT without knowing what HAMT is). The entire payload is still buffered in memory – we track that in File → menu → Download buffers entire file into a blob #1887

In the future:

• we will have dag patch ('ipfs dag patch' kubo#4782), but it will work at the DAG level (too low, does not abstract sharding)
• .. so we need to find a better abstraction for creating ephemeral directories for use cases like this one
  • could be ipfs files patch that works on MFS paths but also on /ipfs/{cid}, becoming true replacement for ipfs object patch
  • ..or ability to have more than one MFS root
  • .. or something else