Implement /convert endpoint for xml/json conversion

[rkorytkowski]

The endpoint should accept POST requests with content in xml or json format and call the convert feature of the validator to transform input to xml or json. It should accept headers:

For Output:

1. Accept: application/json
2. Accept: application/xml

For Input:

1. Content-Type: application/json
2. Content-Type: application/xml

The conversion endpoint could also be extended to support conversion from/to different FHIR versions with URL parameters fromVersion and toVersion.

I'd like to work on this feature once approved.

[rkorytkowski]

Draft PR at #167

[lawley]

https://www.hl7.org/fhir/resource-operation-convert.html provides a spec for this.
It should also handle fhirVersion=... in the Accept & Content-Type headers