Allow filtering by content claim type

[olizilla]

Let's store the can capability name (e.g. assert/equals) in the dynamo record so we can allow users to ask for just claims of a given type.

For the CAR-by-PieceCID api, i only need assert/equals claims, but currently have to fetch all claims for a cid, and filter by claim type after receiving a bunch of claims i don't need.