Add characteristic.type 0..1 CodeableConcept to EvidenceVariable

In May 2021 Connectathon efforts to create EvidenceVariable Resources to define specific interventions for evidence and guideline use there was a desire from the community (University of Berlin) to represent the specific characteristic (eg. dexamethasone) and also represent that the characteristic is a type of intervention. Group Resource uses a code and value[x] pair, while EvidenceVariable Resource uses only definition[x]

We propose to add an optional type element to EvidenceVariable.characteristic to support  expressing the type of characteristic.