Document Platform APIs

[JPercival]

The cql-evaluator needs documentation around the APIs a platform needs to implement in order to support CQL execution and the FHIR operations. These are roughly:

1. FHIR Resource Crud
2. FHIR Searching/Batching/Transactions
3. Terminology Services
4. CQL Services (Caching / Configuration)

Additionally, specific operations could have their own requirements, like job scheduling for PlanDefinition/ActivityDefinition $apply.

[JPercival]

Related: The current interfaces are CQL-centric requiring a fairly deep understanding of what the DataProvider, LibraryLoader, and so on do. Ideally, the Platform APIs would be FHIR-centric with most of the code to utilize them correctly contained internally in the cql-evaluator. This would simplify integration on new platforms. Need to solicit community feedback on what those should look like so that they are as easy to implement as is possible, and also remain generally stable going forward.

[JPercival]

Discussion ongoing here: https://github.com/DBCG/cql-evaluator/discussions/90