Add FHIR xml support

[rkorytkowski]

Evaluate FHIR validator deployed as a microservice with REST interface as a mean to support xml and FHIR validation.

Explore 2 options for FHIR validator:

1. Deploy https://github.com/hapifhir/org.hl7.fhir.validator-wrapper which can be seen at https://validator.fhir.org/
2. Build a REST only wrapper based on https://github.com/hapifhir/org.hl7.fhir.core/

Alternatively check fhir.resources or some universal json to/from xml python converters.

[rkorytkowski]

PR at hapifhir/org.hl7.fhir.validator-wrapper#167