Server-side JSON fetch and processing

[maraf]

• [x] json:fetch, different source for json input mode
  • [ ] Follow caching headers by default Moved to https://github.com/neptuo/is4wfw/issues/527
  • [x] Attribute for explicit minimal cache
  • [x] Provide response status code and possibility to throw error or process the response differently
• [x] json:key without name will enumerate all keys in the current object, available only in the input mode

<json:fetch url="http://api/streams">
    <json:object>
        <json:key name="payload">
            <json:object>
                <json:key>
                    <var:declare name="cameraKey" value="json:key" />
                    <json:object>
                        <json:key name="name">
                            <list:add name="cameras" key-key="var:cameraKey" key-name="json:value" />
                        </json:key>
                        <json:key name="channels">
                            <json:key>
                                <json:object>
                                    <list:add name="channels" key-cameraKey="var:cameraKey">
                                        <json:key name="name">
                                            <list:set name="channelName" value="json:value" />
                                        </json:key>
                                        <json:key name="url">
                                            <list:set name="channelUrl" value="json:value" />
                                        </json:key>
                                    </list:add>
                                </json:object>
                            </json:key>
                        </json:key>
                    </json:object>
                </json:key>
            </json:object>
        </json:key>
    </json:object>
</json:fetch>