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
The other option would be to have the Primo mappers have a common method which generates a collection identifier string, which then gets passed to `Krikri::MD5Minter#create`.