Shared extension for measures using a data element / value set

[eyuasa]

Within the IGs generated by MADFSH, we will need to add extensions to the StructureDefinition to indicate which elements and value sets are used by specific measures. For simplicity to start, we are defining the extensions within the IGs. However, long term these extensions should be shared and therefore defined outside of the specific certification IGs. Determine what the right place is to do that.