JSON/XML PATCH support

**Is your feature request related to a problem? Please describe.**
http://www.hl7.org/fhir/STU3/http.html#patch
http://www.hl7.org/fhir/R4/http.html#patch

**Describe the solution you'd like**
JSON, XML PATCH method is supported as described in the spec.

**Additional context**
relates to #302, #363 

**Failed tests**
`ConnectathonPatchTrackTest`