Feat: dag-json supports pathing

[SgtPooki]

fwiw if we want to close the feature gap, there are conformance tests that aims to ensure dag-cbor an dag-json pathing behaves the same way:

• https://github.com/ipfs/gateway-conformance/blob/63f1d22c97bddf4e84b7bcc4bf1ddcea8530151a/tests/path_gateway_dag_test.go#L353

in practice, dag-json is not used for storage, it is only used as human-friendly representation of dag-cbor, so lower priority

Originally posted by @lidel in #169 (comment)