Reserve Entity ID Range

In networking, It's fairly common to synchronize some pieces of game state. Currently, almost all networking solutions use their own ``HashMap`` or other indirection to map entity IDs between a client and server. While this solution generally works, it adds an unnecessary layer of complexity and makes it harder to reason about networked logic.

I am proposing extending the bevy API to include a ``reserve_entities()`` method, or something to this effect. This would allow you to reserve a contiguous interval of entity IDs, and they would not be used by bevy. This would reduce indirection and complexity in writing networked games, and will likely have other non-networked benefits in other circumstances. The ability to have deterministic IDs is generally a useful pattern, and I think it has a lot of value in Bevy.

In the coming days I will begin working on a naive approach just to get some feedback, but I would love to hear other thoughts in the meantime.