Allow to emit objects not only strings with socketio

[claustres]

Although the code base seems to allow it a validation error is raised (at least with JS scripts) when using the array version of emit including an object item, the problem is detailled in comments of #831.

If anybody can confirm it is a bug or something to be investigated more deeply I would be grateful. I could help with a PR in any case as it would be very useful for the feathersjs community. I wrote an article before on stress testing with socketio and would be pleased to update it by showing how to use artillery to achieve the same thing.

[claustres]

One thing I can see that could be a problem is templating because the emitted object is not a string anymore in this specific case. One possibility should be to recurse the object structure and apply templating on all strings or a defined list of properties coming from the flow definition. Another is the ability to call a processor function like what is done in the http engine 'eg beforeScenario for instance) so that this fucntion could update the emitted object with dynamic values before emission.

[claustres]

Made change in this fork so you can nom emit data like this:

flow: [{
  emit: ['find', 'my_service', { name: 'xxx' }]
}]

Added also a beforeEmit processor function in order to be able to dynamically update the emit args before emitting, it seems to work fine and avoid the requirement to perform templating. However async functions are not supported yet as I am not familiar enough with the code base to do so.

I guess a similar afterEmit processor function should be helpful to check acknowledgment or response for more complex dynamic scenarios than just what is possible now (ie a single match based on a JSON path expression).

[claustres]

Actually added the ability to call validation functions from processor for response or acknowledgment like this:

acknowledge: {
  function: 'checkNavigationResponse'
}

If your function raises an error it will consider the validation has failed.