Get progress in callback for HTTP requests

[Jhonnyg]

Added a new parameter to the option table for the http.request function called 'report_progress'. When report_progress is true, the amount of bytes sent and/or received for a request will be passed into the callback function:

http.request("http://my-address", "GET",
	function(self, id, response)
		if response.bytes_total ~= nil then
			update_my_progress_bar(self, response.bytes_received / response.bytes_total)
		else
			-- handle response like normal
		end
	end,
	nil, nil, { report_progress = true })

Fixes #6486

[Jhonnyg]

unused

[Jhonnyg]

I'm using the test_gamesys.h in the HTTP gamesys tests

[Jhonnyg]

This file is here for two reasons:

• I don't want to mess the current HTTP tests up that was moved into gamesys
• I don't want to put this into the main gamesys test because it relies on a http server running, and the gamesys test is getting a bit out of hand size wise

[Jhonnyg]

I want to reuse the script test server, so I added a simple plugin mechanism to it

[Jhonnyg]

I reuse this in the gamesys http tests now

[britzl]

Lua API wise it looks good I think. Backwards compatible and easy to use. I'll let @JCash comment on the other code changes.