feat: Mater convert, wrap contents in unixfs dag

[cernicc]

The client retrieval system expects a car file with dag information so that it knows which blocks to request from the server. The current issue is that when the client receives a root node. It doesn't know which blocks are next in the tree to be requested.

The current convert command converts contents to a car, but it doesn't encode any dag nodes metadata. That should be changed so that the nodes that are written to car also at least contain links to other nodes.

[jmg-duarte]

Even though this is no longer relevant for retrieval right now, I'm keeping this one open since this is a difference between mater and go-car that we can address