Update to define vaccine codification by { value, codesystem }

[bradhead]

Not all sources will be offering pre-mapped vaccination codes to the codes of choice for this vaccination certificate. I'd prefer to leave the task to the verifier to understand source codifications as they have a vested interest in comprehending the codification. This is less effort for the sources, who may then offer the vaccines codified in their 'native' system. For Canada, that is generally the Drug Identification Number, or DIN with a code system as a HL7 FHIR URI. These DINs are resolvable to human-readable content via APIs that are managed by the Canada Gov and the CVC. Our sources of truth for vaccincation records in Canada do not generally store the WHO's ATC code, nor any other code than DIN, with some ICD and/or SNOMED-CT extension codes as exceptions.