add subpart support

[squidgetx]

Support minting ARKs that are a child of an existing ARK.

I guess we would pass in a parent_ark parameter in the minting ARK path and just append the generated ID to it before saving it in the DB.

Unclear that we even care about checking that the parent ARK exists in the database, since in theory different resolvers could resolve the parent.

Prioritization of this is not clear right now as it's not something we discussed with Frick but seems like an obviously useful feature.