Implement generic physics backend support

[zbuc]

This adds some groundwork to be able to specify custom physics backends.

Existing code should work without changes and if you want to use a custom physics backend it should be possible to insert a PhysicsBackend::Custom resource with the desired custom backend.

[adrien-bon]

Hey! I just updated your change, I hope you don't mind.

What I did:

• I changed a bit the mechanism. Instead of using a trait, which I feel is quite complicated, we now use an event. It also allows to have a standard system to handle colliders spawn: this way, we can get all Bevy "standards" resources. For instance, to create a mesh. It would have been a bit more complicated with the previous solution.
• you were using a TextBundle in your example which is a UI component and spawned "on the screen" and not "in the world". it now uses a 2D mesh which seems more appropriate.
• I fixed an issue in the base code: it was using a TransformBundle for tiles and object, which does not have a Visibility component. They now use a SpatialBundle which have it. This was preventing the new collider entity to inherit Visibility.
• Update changelog / readme and fixed some compilation issues / clippy warnings.

I'll let you review and let me know what you think. I hope the changes I made make sense for you :)

Thanks!

[zbuc]

Looks good to me, thank you for the review! I am new to Bevy and still unfamiliar with much of it so I appreciate the suggestions.