Spike: Research support for required extensions for $apply

There are a number of extensions defined in various IGs for PlanDefinition $apply. We need to go through and determine which extensions we should support for our $apply, and design a solution for extensible extension support in the PlanDefinitionEngine component

IGs include Da Vinci DTR, CRMI, SDC, CPG, and so on.

https://hl7.org/fhir/uv/cpg/artifacts.html#extension-index
http://build.fhir.org/ig/HL7/sdc/artifacts.html#structures-extension-definitions

Considerations:

• Where, when, and how extensions are applied (are there common semantics among extensions)

Some possibilities for a design:

• Dynamic Action registry
• Dynamic RequestGroup/CarePlan mutator registry
• Simply carrying extensions forward through processing

Acceptance Criteria:

• We've documented a set of extensions that we think we should support
• We've documented a design for supporting these extensions.

Start by reviewing the PlanDefinitionEngine component and the extension definitions in the linked CPG and SDC implementation guides, then investigate the referenced Da Vinci DTR and CRMI guides. Document which extensions should be supported, their application semantics, and a design for extensible support; completion means both documents address the acceptance criteria.