Spike: Research support for required extensions for $apply

[JPercival]

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:https://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.

[JPercival]

TODO: JP to find the extension master list from the autobuilder.