Add endpoint for FHIR resource operation

[dotasek]

If possible, we should make an endpoint compatible with Operation $validate on Resource as defined in the FHIR specification:

http:https://hl7.org/fhir/resource-operation-validate.html

[costateixeira]

I was wondering if it would make sense to encapsulate it in a way that it gets called when we simply do a

POST /?validate
Accept: application/fhir+json; fhirVersion=4.3.0
Content-Type: application/fhir+json; fhirVersion=4.3.0

{ resource json here}