Run criteria for update_system

[hyranno]

I want to provide ways to configrate when to update behavior tree.
As being discussed in this issue, Bevy itself has no official interface to do it for plugins now.
Fortunately bevior_tree has only one system, so making it configurable is not hard.
I have two options here.

1. Take State as generics type parameter, then use it for run criteria.
2. Take run criteria and use it.

Former is popular, latter is more configurable.
It may be difficult to run system for setup like OnEnter(state) with the latter, so other plugins may prefer the former.