Add a lax version of `RelationshipTarget` as a super trait.

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

An abstraction of "references multiple other entities" is very commonly needed, having it in bevy is nice. `RelationshipTarget` is overly specific and does not fit every use case.

## What solution would you like?

Add a super trait to `RelationshipTarget` called `EntityReferences`.

```rust
pub trait EntityReferences {
fn get_entities(&self) -> impl IntoIterator;
}
```

This works with most use cases from `Vec` to `Option`.