digest: add `DynDigestWithOid`

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