FHIR consumer: support outgoing HTTP headers

[qligier]

I would like to set HTTP headers in a FHIR response when using a FHIR consumer, e.g. for Trace Context support, by using Constants.HTTP_OUTGOING_HEADERS.

I tried to check the code but could not find a place where Camel's exchange would exist alongside Hapi's HttpServletResponse, to map the headers from one to the other.
Is there any way to do it?

[unixoid]

@qligier Please review and test. What I did not manage to implement is the propagation of response HTTP headers into the producer route in case of queries -- see e.g. org.openehealth.ipf.platform.camel.ihe.fhir.chppqm.chppq5.ChPpq5Test#test2. But the headers are there, you can see them when you set logging level of the FHIR logging interceptor to INFO.