[Feature Request]: add possibility to set arbitrary HTTP headers for the response of web services

### What would you like to happen?

The "Web Service" metadata object only allows to configure the "Content type" header of the HTTP response (however, suffering from https://github.com/apache/hop/issues/5425), but has no other options.

In order to enable to set the filename of the downloaded files, we would need to option to set "Content-Disposition" header, see also https://stackoverflow.com/a/9195376.

Even better would be the possibility to set arbitrary HTTP headers, similarly to how it can be done for the HTTP requests in the "Headers" tab of the "REST client" transform.

### Issue Priority

Priority: 3

### Issue Component

Component: Hop Server, Component: Metadata

Start with the Web Service metadata object and the Hop Server response handling, then compare them with the REST client transform's Headers tab. Trace how response metadata is configured and emitted. Done means web services can set Content-Disposition and other arbitrary HTTP response headers, not only Content type.

Written by the indexing model from the issue text.