Support tagged digested references

[glyn]

dkregistry::reference::Version does not accommodate tagged digested references. Although these are not particularly common, they are valid.

[lucab]

Indeed. Related, it would probably be a good idea to first address this TODO item for parsing references:

dkregistry-rs/src/reference.rs

Line 156 in cb22b53

// TODO(lucab): investigate using a grammar-based parser.

[glyn]

Perhaps. One starting point for a reference grammar is in a code comment; I am not aware of a grammar in the OCI specs. The testcases clarify the intention though.