Add specification for queries for verifiable CAR requests

Co-authored-by: Henrique Dias <hacdias@gmail.com>
Co-authored-by: Alan Shaw <alan.shaw@protocol.ai>

This adds the IPIP document for 402 and documents some of
prior discussions, decisions, and known testing to-dos from
GitHub and Slack conversations

Additional work is needed, but should be a good start.

This change incorporates feedback from Adin, Rod and Juan:

- bytes: #402 (review)
- car-scope: #402 (comment)

I really hope these names will be good enough, but I am running on
artisan, recycled electrons so can do this all day :-)

We have no spec for signaling this, we may add it as opt-in later
#402 (comment)
#402 (comment)

Problem identified in
ipfs/gateway-conformance#56 (comment)

Rationale:
#402 (comment)

#402 (comment)

Discussions in #402
illustrated deeper problem with CAR determinism, and
we made a decision to remove its aspects from IPIP-402.

Ref.
#402 (comment)

#402 (review)

This backports CIDs and CARs from gateway conformance to the IPIP-402,
and provides some basic hints on how each fixture should be used.

#402 (comment)

#402 (review)