Add a simple `EntityMapper` struct for use in networking

[alice-i-cecile]

          That's the behaviour I'd prefer to have for networking, as I don't want/need the complexities associated with the existing EntityMapper. (Having to use `EntityMapper::world_scope()` and needing access to `&mut World`)

I will remove this from the PR as users can implement it themselves

Originally posted by @cBournhonesque in https://github.com/bevyengine/bevy/pull/11428#discussion_r1467883777

[Shatur]

I'm not sure if it's possible to have a general enough struct for networking. In replicon I have different mapper for events and components, for example. Since it's just a few lines of code, I would left it for libraries to implement.

[alice-i-cecile]

Mhmm. Might be more useful as an example? It's hard to say: networking is a nasty beast.

[Shatur]

Yes, an example would be good. But maybe worth to put into rustdoc to the trait description?

[alice-i-cecile]

Yeah, I like this a lot as a doc test.