Add the resourceType field to Avro records

[bashir2]

This is a follow-up of #1003 and in particular this comment. Instead of trying to record the resource-type in the id field, we should add a resourceType field to the Avro representation of FHIR resources. Although this field is present in the JSON representation of all FHIR resources, but it seems to be missing from the StructureDefinition files and also the documentation.

[bashir2]

This is not high priority for now (hence P3) because conversion to HAPI objects and then JSON encoding, adds the resourceType field.