support optional/required "Host" header when the serviceUrl is proxied/ip/et al

When within a corporate/proxied/tunnelled/containerised environment the service-url may well not be the "Host" that is required.

This allows support for specifying an optional "Host" header to minimise environment requirements:
 * No need to use a proxy
 * No need to use a dns override
 * et al

serviceUrl can be ip/other host/et al. Though the request will add any configured "Host" header to the endpoint.

Example:
ServiceURL is a tunnel from a container to an own proxy. The container has no knowledge of the "proper" endpoint "Host" header. Using this extra option can ensure the correct "Host" is sent.