Expose the response body after creating ExternalMedia channels

[reecewithnospoon]

StageExternalMedia calls post with nil for the response, so the response body is discarded.

In my case, I am multiplexing ExternalMedia packets to a single UDP port on my Go server.

If I don't get the UNICASTRTP_LOCAL_PORT ChannelVar from the 200 OK response, then I cannot know which session the incoming UDP packets belong to.

I need access to the response body after creating an ExternalMedia channel. Currently I can only do this by building my own http request and unmartialling the JSON manually.

Calling ChannelHandle.Data() does not reveal the UNICASTRTP_LOCAL_PORT which was returned at the time of creating the ExternalMedia channel, so we can only get this information from the initial response body.