Support DEQM supplementalData extension

[brynrhodes]

The evaluate-measure operation currently uses a pre-adoption extension for the supplementalData element introduced in MeasureReport in R5. However, the DEQM IG has defined an R4 extension in the IG for supplementalData. We should use the extension specified in DEQM rather than the pre-adoption extension spelling for this capability:

https://hl7.org/fhir/us/davinci-deqm/StructureDefinition-extension-supplementalData.html

[c-schuler]

Investigation shows that the supplementalData extension landscape has evolved since this issue was filed:

• DEQM STU4 (v4.0.0): Defined a custom extension http://hl7.org/fhir/us/davinci-deqm/StructureDefinition/extension-supplementalData (what this issue recommended adopting)
• DEQM STU5 (v5.0.0): Dropped the custom extension and switched to an R5 cross-version mapping: http://hl7.org/fhir/5.0/StructureDefinition/extension-MeasureReport.supplementalData

However, https://jira.hl7.org/browse/FHIR-50091 identifies a structural problem with the STU5 approach: because the cross-version mapping comes in as an extension, it cannot have both a value[x] and sub-extensions (criteriaReference, description) as siblings. Those sub-extensions need to be on value[x] itself, which requires publisher exemptions.

Our codebase currently uses http://hl7.org/fhir/5.0/StructureDefinition/extension-MeasureReport.supplementalDataElement.reference (a different R5 pre-adoption URL that predates both approaches).

What is the recommended course of action here?

@brynrhodes

[brynrhodes]

FHIR-50091 is published in STU5, so we should produce following that approach (namely, use the cross-version extension with criteriaReference and populationDescription added to the value element.