Generate HTML preview for DAG-CBOR

[lidel]

Filling this for completeness, and also as bare minimum to support https://dasl.ing/, but with lower priority than other things like #342

If request for DAG-CBOR CID is made with Accept header that includes text/html, we should generate HTML preview that

• informs user that CID is DAG-CBOR
• have option to download CBOR as a raw block (?format=raw, requires Trustless responses: ?format=raw|car|ipns-record #356)
• preview of CBOR document with clickable links to other CIDs (CBOR Tag 42)

Prior art in boxo/gateway: https://ipfs.io/ipfs/bafyreicnokmhmrnlp2wjhyk2haep4tqxiptwfrp2rrs7rzq7uk766chqvq/

Note:

• MVP does not have to be styled with CSS or have links to anything other than Raw Block
• we do not have conformance for this, but we should add one, that confirms CBOR Tag 42 link is present and clickable