Implement tooling to document hardcoded CIDs

See the changes in https://github.com/ipfs/gateway-conformance/pull/87

We used fixtures + libraries to generate expectation values (like an expected payload or an expected CIDs)
But in some (most?) cases, this relies on the fact that the library itself is correct.

Take a test that verifies the ordering of blocks in a CAR produced by a gateway, if we use go-ipld-xyz to load the car and generate the array of blocks, then we test against the library, not against the spec.

The other option is to hardcode results (like arrays of CIDs) which is okay up to a point where we have dozens of long lists of random strings and one test breaks or we have to update it.

Ideally we'd have some way to describe CIDs & other payloads like documentation and load these in the code, similar to how we do DNS link configurations.