Need support for sending arbitrary headers in REST Call template

[theovassiliou]

As a test case writer I would need a functionality that enables me to send a specific header value.
Example:
type Request {
string challange } with {
encode (challange) "header/string" ;
variante (challange) "'Challange";
}
send(Request: { challange := "xyz" }

In the http message I would expect to see:
....
Challange: xyz
....