Make fuels event listeners accept async callbacks

[nedsalk]

Currently, the FuelsEventListener type returns void. We should update the return type to Promise<void> | void because being able to pass in async callbacks comes in quite handy sometimes. Besides this type, we should update the onFailure listener as well.

This would entail always awaiting the listener callbacks. As of now, sometimes we do it, sometimes we don't.

[nedsalk]

@arboleya I don't know if I understood you correctly in #3504 (comment), so I'll just assume that you meant making it always async so that the return type is Promise<void> only. This would be a breaking change - people's compilations/linters would fail because their synchronous callbacks returning void can't be assigned to the new type returning Promise<void>, hence why I described the return type to be Promise<void> | void in the issue.

[arboleya]

You can use Promise<void> | void, no problem.