hapi-fhir-cli - Add option to pass request headers to target server

Add header-passthrough option to enable defining additional http headers to be aded to command-generated http request.

This option is required by users defining custom interceptors requiring additional headers. 

Option must support multiple parameters defining different header keys or multiple values for the same key.

Version: HAPI FHIR 5.4.1 (Pangolin)

