Entity Markers

## What problem does this solve or what need does it fill?

A Marker trait that can mark an entity. example

```rust
#[derive(Marker)]
pub enum Players {
Red,
Blue,
Green,
}
```

then we can assign an entity with a marker
```rust
commands.spawn(PlayerBundle::default()).mark(Players::Red);
```

and we can easily query players
```rust
fn add_items_to_red_and_blue(red_query: Query>, MarkedAs>>) {
// ...
}
```

## What solution would you like?

An Marker that works like a Resource but holds a EntityID and the Marker can be used in Queries.

## What alternative(s) have you considered?

Manually make Resources holding EntityIDs and then using a normal query and using .get with the entityID from the resource.