Special serve logic for SZDT / CAR + MASL

[gordonbrander]

• CAR/MASL as envelope format szdt#7

Advantages:

• Gives us a place to put content-type and other metadata
• Read select metadata from CAR headers (e.g. content-type), use for HTTP headers

Prior art:

• IPFS gateways do some special stuff for application/vnd.ipld.car
  • https://specs.ipfs.tech/http-gateways/trustless-gateway/#accept-request-header