More Complete Retrieve Support

There are several retriever-associated improvements or bugs I've noticed so far, and wanted to brain dump them all in one place. We can break these out into separate issues, or use this issue along with the GitHub subtasks below to track changes and discussion in one place. These are in rough priority order in my mind but should be assessed further. This is not an exhaustive list, and folks should feel free to add more.

- [ ] The correct default comparison _system operator_ should be used for filtering (this will also help us eliminate a specific FHIR proto dependency). More [here](https://cql.hl7.org/02-authorsguide.html#retrieve).
- [ ] Support CodeSystems and single Code inputs for the terminology filtering
- [ ] Support an explicitly supplied [codeComparator](https://cql.hl7.org/19-l-cqlsyntaxdiagrams.html#retrieve) operator
- [ ] To look into: We might want to implicitly convert the `terminology` expression to ValueSet, CodeSystem, or a single Code (e.g. it might be legal for someone to supply a FHIR.ValueSet here that should be implicitly converted to ValueSet).
- [ ] We may want to expand the Retriever interface itself to allow passing of more information (e.g. constraints) to the Retriever implementations, which could lead to retrieve efficiencies.
- [ ] We may want to revisit having an R4 specific proto in the Retriever interface and pick something more generic at that interface.