Remove static keyword from SubscribingHandlerInterface

I'm wondering why it is that the SubscribingHandlerInterface describes the getSubscribingMethods as static. From what I can see, all the code interacting with the method is acting on an instance of the handler.

This PR removes the static keyword. It is very much a BC break because the interface has changed.

My use case is allowing for configurable handlers based on configuration done to the instance. For example, you could configure the methods or types that the handler supports.

If there is reason for the current design, please forgive my naivety -- I would very much appreciate your thoughts here!