Add legacy MD5 Minter

Submitting this for discussion.

Interested in ideas about more robust testing of edge cases for existing ID's. This needs to be able to replicate https://github.com/dpla/ingestion/blob/develop/lib/akamod/select-id.py#L55
I had considered handling it in the harvester, but I'm not sure what is best.  It's going to be slightly awkward however it is handled and I'm leaning toward stowing the awkward in the `MD5Minter`; that way all the legacy logic is in one place.