Keyd relationships

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

I've had some situations where i had to create multiple 1:1 relationships because the relationship itself had some semantic to it, like `BodyPart(Entity)` with `T` being `Hand`, `Head`, `Foot`. Having a type of `RelationshipSource` that can take a key would centralize all of these variants into one 1:N relationship.

## What solution would you like?

A `HashMap` (or similar) `RelationshipSource` where you could do

```rust
#[derive(Component)]
#[relationship_target(relationship=BodyPart)]
struct Body {
parts: HashMap,
}

#[derive(Component)]
#[relationship(relationship_target=Body)]
struct BodyPart {
part: Part,
body: Entity
}

enum Part {
Head,
Hand,
Foot,
...
}
```

## What alternative(s) have you considered?

Keep the multiple 1:1 relationships

## Additional context

I use this to have a quick access to bones in a armature to easily attach items to the bone