Add support for FHIR Server retriever

[evan-gordon]

Interest was expressed in this zulip thread to add support for using the engine with FHIR Server based retrievers.

We currently have the following implementations:
A local retriever: https://github.com/google/cql/tree/main/retriever/local
And a GCS retriever: https://github.com/google/cql/tree/main/retriever/gcs

Each of these implement the retriever interface defined in https://github.com/google/cql/blob/main/retriever/retriever.go

Side note: we may benefit from additional docs giving more insight into developing a custom retriever.

[preston]

For context, I'd made the request on behalf of the NIH-funded "SHARES" grant program at Arizona State University, on which I'm a co-investigator. We have a number of deidentified patient records in HAPI, and are writing a custom CQL library to aid in the processing and enforcement of "substance use" data and FHIR Consent documents under 42 CFR Part 2. Couple architectural slides: https://www.asushares.com/technology