Include an element for FHIRPath Version in ViewDefinition

[vneilley]

The current proposal has support for ViewDefinition business version and resourceVersion, however, there does not appear to be an element for FHIRPath version used to define the view. As FHIRPath continues to mature, serves may need to know what version was used to create a view to ensure view consistency. Furthermore, if a view uses a recent FHIRPath version, it may not be supported by all servers if there is a lag/difference in adoption

[rbrush]

We do need to fully specify the set of FHIRPath used -- a rough attempt at that is here. At the same time, as I understand it FHIR itself is tagged to specific FHIRPath versions, so if we use FHIR r5 (for instance), that implies we're using FHIRPath 2 as specified at https://build.fhir.org/fhirpath.html to me.

But I wouldn't object to adding an optional field if that needs to be overridden for some reason as well.

[vneilley]

From an implementation perspective, we are considering allowing some flexibility in the FHIRPath version that is used so migrations or backwards support for more complex FHIRPath querying to happen in previous versions of resources.

From a usability perspective, having canonical ViewDefinitions that can run on federated or disparate data/FHIR stores is extremely powerful, but may mean an R5 store is running an older version of FHIRPath to ensure consistency across the federation (portability)