[8.x] Make Illuminate\Http\Client\Request macroable

[funkjedi]

This PR makes the Illuminate\Http\Client\Request class macroable.

Having the Request macroable will allow for the elimination of excess boilerplate code when faking Http requests with body content other than Form or JSON, XML for example.

Request::macro('xml', function () {
    return CustomSuperDuperXmlParser::parse($this->body());
});

Http::fake([
    'example.com/*' => function (Request $request) {
        $this->assertSame($request->xml()->someProperty, 'some value')

        return Http::response();
    },
]);