Implement Hoverfly configuration by PAC file

[benjih]

I have added a new endpoint to Hoverfly called /v2/api/hoverfly/pac. A user can GET or PUT a PAC file from/to this endpoint. Putting a PAC file on this endpoint will allow Hoverfly to dynamically set the upstream proxy based on the result of the PAC file. This should fulfill issue #762 and older issue #583.

PAC files are Javascript functions that are usually executed by a client like a web browser.

function FindProxyForURL(url, host) {
	// our local URLs from the domains below example.com don't need a proxy:
	if (shExpMatch(host, "*.example.com"))
	{
		return "DIRECT";
	}

	// URLs within this network are accessed through
	// port 8080 on fastproxy.example.com:
	if (isInNet(host, "10.0.0.0", "255.255.248.0"))
	{
		return "PROXY fastproxy.example.com:8080";
	}

	// All other requests go through port 8080 of proxy.example.com.
	// should that fail to respond, go directly to the WWW:
	return "PROXY proxy.example.com:8080; DIRECT";
}

I have achieved this by using jackwakefield/gopac, which uses github.com/robertkrimen/otto, A Javascript interpreter written in Golang. If a PAC file is provided, it will get parsed and executed in by the interpreter and then with the result, will configure a HTTP client correctly.

There are functional tests in this PR proving Hoverfly retrieving responses from two different Hoverfly instances running in simulate mode, with the routing between the two instances determined by a PAC file.

[benjih]

Forgot to mention, as I needed to pull new dependency in, things obviously went sideways, hence all the vendor changes.