Specify shim output in the trigger descriptor

[fcastill]

Current behavior:

The output of the shim builds of a trigger varies depending on the implementation of the trigger. Because of that the Web UI cannot provide proper support for it as it doesn't know where to look for the file to download.

For example the CLI trigger saves its output to bin/${app-name} but the AWS lambda trigger saves it output to bin/handler.zip (until recently it was src/handler.zip).

Expected behavior:

The trigger's descriptor.json can declare a field like:

{ 
 "shimOutput": "bin/handlers.zip"
}

Which the UI can use to find the output file.

What is the motivation / use case for changing the behavior?

Supporting shim builds in Flogo Web UI