feat: Mater content extraction from unixfs dag

[cernicc]

Extend our content extraction by enabling mater to also be able to extract content from .car files that store unixfs dags.

The current extraction only works in cases when the car file contains ordered blocks with arbitrary content. The blocks are expected to be in the correct order. The extraction then iterates over all blocks in sequence and extracts that raw content. This works great for many cases, but in our case we'll have content wrapped in an unixfs dag, the extraction process should know how to walk that graph and extract the content.