Send Bevy events when things happen

[rparrett]

It would be very useful for Bevy events to be sent when Jornet calls succeed or fail.

A couple example use-cases:

• I only want to show the leaderboard at the end of a game after the player's score has been added.
• I want to create my own player names and persist the resulting Player so players don't get a new name every session.

To accomplish these things, I need some more feedback from Jornet than "leaderboard was changed."

Success/Failure events for each call would be sufficient for my needs, although I could imagine more data also being useful (e.g. returning some sort of "score identifier" when sending a score succeeds)