Add vaccine serial number to AU Base Immunization

[dt-r]

This element should be added to AU Base Immunization as an optional extension instead of listed as 'potentially useful' in the intro.

Downstream IGs that derive from AU Base Immunization or AU Core should be able to pick this up for free if they need it.

[dt-r]

See https://build.fhir.org/ig/hl7au/au-fhir-base/branches/GitHub825/StructureDefinition-au-immunization.html