Extend unixfs builders to optionally produce file references instead of blobs - hackish version

[rklaehn]

We don't know yet where this will happen (on the cli or on the store side), but to turn the store into a store that optionally just stores metadata, we do need to extend the builders.

[rklaehn]

This is going to be fun. Not much you don't have to touch in unixfs...

[rklaehn]

So here is the hackish WIP version: https://github.com/n0-computer/iroh/tree/block-references