Introduce an Enum for "diagnosis certainty"

Introduce an Enum for what we've historically called "diagnosis certainty" and FHIR refers to as ConditionVerificationStatus (https://www.hl7.org/fhir/valueset-condition-ver-status.html). We will only support a subset of the FHIR statuses:

enum ConditionVerificationStatus {
    PROVISIONAL, // was called "Presumed" in the original EMRAPI module implementation
    CONFIRMED
}