Extract PieceCID from ContextID

[bajtos]

To allow Spark to link deals to content advertised to IPNI, SP software like Curio creates ContextID from Piece info. This is provides an alternate way for extracting PieceCID from IPNI advertisements.

We need to enhance piece-indexer to support both options: PieceCID extracted from ContextID, PieceCID extracted from Graphsync metadata.

Spec:
https://github.com/CheckerNetwork/FIPs/blob/frc-retrieval-checking-requirements/FRCs/frc-retrieval-checking-requirements.md#construct-ipni-contextid-from-piececid-piecesize

Optimisation:
ContextID values following the spec above start with the prefix ghsA. If the ContextID value does not start with this prefix, we can skip it (there is no need to try to do base64 and CBOR decoding).

See #117 for an example index provider that uses this new format.

Here is the place where we are extracting PieceCID from Graphsync metadata, we can add the new ContextID-based PieceCID extraction there:

piece-indexer/indexer/lib/advertisement-walker.js

Lines 273 to 281 in 684bdc0

	const meta = parseMetadata(advertisement.Metadata['/'].bytes)
	const pieceCid = meta.deal?.PieceCID.toString()
	if (!pieceCid) {
	debug('advertisement %s has no PieceCID in metadata: %j', advertisementCid, meta.deal)
	return {
	error: /** @type {const} */('MISSING_PIECE_CID'),
	previousAdvertisementCid
	}
	}