Expose input action from enhanced input component delegates

From a delegate there is no easy way to access the action that triggered the delegate. Whilst this is mostly inherent (an OnJumped handler implies the Jump action) it would be better to provide direct access to allow sharing of handlers between multiple actions, and reduce delegate handling work for the user.

Suggest leaving the simpler delegates as they are, but indirectly modifying the delegate for FEnhancedInputActionHandlerInstanceSignature by exposing FInputActionInstance::SourceAction as read only for both native and BP as well as exposing an action output node on K2Node_EnhancedInputAction - which already tracks the input action internally.

1. Enable enhanced input plugin
2. Create an Input Action Asset
3. Create an Input Mapping Context and map the input action asset to any key
4. Create a custom player controller
5. Add the mapping context to the player controller
6. Place an event for your input action (See attached image)

Result: There is an input action pin on the event that is placed