digest: add `DynDigestWithOid`

[tarcieri]

It'd be nice to have an object safe trait for obtaining the OID of a DynDigest. Something like:

pub trait DynDigestWithOid: DynDigest {
    fn oid(&self) -> ObjectIdentifier;
}

Separately that could probably use a trait like DynAssociatedOid in the const_oid crate, which ^^^ could be built on as a supertrait bound + blanket impl.

[newpavlov]

Could it work with fn oid(&self) -> Option<ObjectIdentifier> on DynDigest or do you need to enforce OID existence at compile time?

[tarcieri]

It would certainly be nice for it to work infallibly when needed