Tree context injection

[darthLeviN]

This is a very useful feature. It exists in react native (via useContext). however it doesn't exist in the game engines that i have seen so far (for example godot).

It is not a good idea to implement such system by plugins as it's susceptible to data races.

My proposals is that app/world would have a method called register_context where you pass a specific component type. and then any entity will be able to use
get_context<C> in commands, world.
and also it would be available in queries using &Option<Context<C>> or &mut Option<Context<C>>

for now i have implemented some traits to search in the tree whenever i need them, but that's not good for performance for heavy usages. Example usages :

• A gun is a child of a character, it needs to access specific character stats. Bevy is an extremely good candidate for strategy games so this will definitely come up.

Implementation of context should be optimized in World with specific map for each context.
If memory usage is a concern we can go with a more process intensive kind of map that only tracks the entities that have the ContextUser<C> component. every time such component is spawned, the entity will be tracked for such context uses. Insert/Remove will become less efficient but it's going to have less memory usage.