Allow to customize serverUrl while using rocketSend step

Today we can only use one RocketChat instance defined in general settings to send notifications. In our team we use multi RocketChat instances so we cannot use this plugin.

Would be nice if we can define a serverUrl directly in rocketSend step.

Example :

rocketSend serverUrl: 'https://myrocket.example.com', channel: 'general', message: 'My message'

Thanks!