Extract sharding awareness to UnixFS

[schomatis]

The current state of MFS and its coupling with UnixFS don't make this a high priority issue; this is only valuable in connection with ipfs/kubo#8106: if we can refactor this code outside of the repo we can avoid adding more sharding logic here that just extends the technical debt.

MFS should deal only with the UnixFS Directory interface and not with its (basic and HAMT) implementations:

go-mfs/dir.go

Lines 374 to 385 in 9c9a0fe

	func (d *Directory) addUnixFSChild(c child) error {
	if uio.UseHAMTSharding {
	// If the directory HAMT implementation is being used and this
	// directory is actually a basic implementation switch it to HAMT.
	if basicDir, ok := d.unixfsDir.(*uio.BasicDirectory); ok {
	hamtDir, err := basicDir.SwitchToSharding(d.ctx)
	if err != nil {
	return err
	}
	d.unixfsDir = hamtDir
	}
	}

go-mfs/dir.go

Lines 166 to 167 in 9c9a0fe

	case ft.TDirectory, ft.THAMTShard:
	ndir, err := NewDirectory(d.ctx, name, nd, d, d.dagService)

The tradeoff of removing this here is adding another layer of indirection in UnixFS that can transition from one implementation to another when needed (as MFS does here).