Discover what versions a server supports

Using the [FHIR Version Mime Type Parameter](http.html#version-parameter), a server can support [multiple versions on the same end-point](versioning.html#mt-version). The only way for client to find out what versions a server supports in this fashion is the $versions operation. The client invokes the operation with no parameters. and the server returns the list of supported versions, along with the default version it will use if no fhirVersion parameter is present