Kubo's filestore extension for enabling the --nocopy option is pretty cool, so I thought it would be great if we could have similar functionality for Helia thorugh the blockstore and UnixFS, so this PR is my attempt to get something along these lines kickstarted.

Based loosely on the Filestore Specification and the Go Implementation.

This works by wrapping an existing blockstore to manage blocks and uses a datastore to keep track of links to the filesystem.
There might be need of changes to Helia's UnixFS module for an option of nocopy to make good use of it (if a filestore blockstore is in use).

This is currently missing any tests.