Allow EvidenceVariable.characteristic.definitionReference to reference Any resource

EvidenceVariable.characteristic.definitionReference is currently datatype Reference(Group | EvidenceVariable). However, in actual examples to represent specific dosage for interventions in reporting evidence for guideline support, we are using Reference(ActivityDefinition) to re-use the structure for dosage expression. We would like to extend the element to reference any FHIR Resource that may represent an evidence variable characteristic.