Plugin interoperability

[Bluefinger]

RNG sources are not just needed at the game level, but also the plugin/crate level. How plugins should manage accessing RNG sources and creating them is an important, given these could interact badly or introduce indeterminism into a game when the developer is trying to achieve determinism in all or any area of importance.

What should be established is patterns or best practices for plugins making use of bevy_rand as their RNG source to do so in a way that can be:

1. Seeding be set/controlled at the game/app level.
2. Fallbacks to using system/thread local entropy or internal seeding within the plugin.
3. Ensure that plugin sources will not interfere with each other.