Add support for FHIR Server retriever

Interest was expressed in [this zulip thread](https://chat.fhir.org/#narrow/stream/179220-cql/topic/Google.20Open.20Source.20Blog.20Post/near/463485620) 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.