Suggestion: Encode request objects as IPLD CAR files.

Was revisiting this spec and I noticed that IPLD (IPFS) is being used in a kind half baked way. Some of the data included in the "request object" is assumed to also be encoded as an IPLD DAG-CBOR object. I believe we can actually encode the entire request object as a set of IPLD objects (e.g. the `authorization` object is already valid DagJOSE). These IPLD objects can then be put together in a single [IPLD CAR file](https://ipld.io/specs/transport/car/carv2/) which can be easily transmitted over the wire as bytes encoded in any preferred manner.

This is mostly a suggestion on how IPLD could be used more natively. In the way the spec is written currently I'm not sure exactly what value using IPLD adds.

Sidenote, [DagJOSE was merged and released in go-ipfs](https://github.com/ipfs/go-ipfs/releases/tag/v0.11.0) :tada: