Make ServerBaseUrl Config Property of FHIR Server Optional

[hhund]

The config property dev.dsf.fhir.server.base.url of the FHIR server is usually not necessary and should be made optional. By setting Host, X-Forwarded-For, X-Forwarded-Host, X-Forwarded-Proto and X-Real-IP headers in the reverse-proxy, the external URL is available to the backend server for example via UriInfo.getBaseUri().

[hhund]

Removing the base url config parameter and using headers from a reverse proxy does not work when the server is accessed via different domain names. But we could make the base url config parameter optional, I will update title and description of this issue accordingly.