gateway: car-* request query parameters

[hacdias]

For ipfs/boxo#604.

[lidel]

Thanks!

Rebased and clarified how they interact with Content-Location.

For anyone following:

• Documenting use of Content-Location in gateways: document Content-Location #471 allowed us to introduce these query parameters safely, in a way that is backward compatible (Accept header taking priority over format query parameter, Content-Type of response being the final source of truth for the client).
• No IPIP as these CAR content type parameters were already present in Accept and Content-Type. Documenting their query versions is a formality to allow exposing them in Content-Location, which in turn allows implementations to ensure unique cache key for requests with parameters other than the implicit defaults.
  • tldr: this allows gateways to cache different CAR response variants without overriding each of them