Expose an api to check whether a compiled Siddhi app is stateful or not

[pcnfernando]

Description:
At the moment, when SiddhiApp is parsed using SiddhiCompiler.parse(), it provides you with the definition maps and execution elements.
In order to check whether the app is stateful, we need to iterate through all definitions and stream handlers and categorize depending on whether we have windows, patterns and such stateful cases manually.
Even in that case, we might miss stateful function usages since that information is not available.

Affected Siddhi Version:
4.x, 5.0.2