Add `emitter` to flow extensions

Summary While hook and endpoint extensions have the ability to emit custom events, operations (Flow-Extensions) are the only server-side extension type which isn't able to emit custom events. This makes it impossible to trigger other extensions events from within a flow. From a technical POV I don't see any problem with an operation emitting an event. I assume the emitter has never been to the context, as the operation technically wouldn't be able to listen to other events. I still think it should be added to operations, an operation should still be able to trigger other extensions events. Basic Example Having a custom logger extension which receives log-items by hooks the extension wouldn't be able to call it. Motivation Allow an flow-operation to emit custom events which can be listened by own extensions. Detailed Design Add the emitter to the operations context. Requirements List Must Have: • Add the emitter to the operations context C…