Networking observers

[Shatur]

I think we should make them networked, but I'm planning to wait for the next Bevy release to let them "bake":

• When James initially implemented observers they had their own separate concept but Cart made the decision that in the long run it makes sense to unify them. Relevant PR: Support Triggering Buffered Events bevyengine/bevy#13953. We are just in a bit of an awkward half-way state.
• There are plans for statically differentiating between an "entity event/observer" and a "target-less event/observer" (see cart's comment under this video).
• There is a critical bug that prevents me from using observers (and hooks) in my game: Observers and hooks doesn't pair well with mapped entities bevyengine/bevy#14465.

After stabilization it will be more clear how they fit in networking.

[Shatur]

Closed by #398.