Immutable relationships

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

Currently when trying to implement relationships on immutable components bevy will complain that the components need to be mutable

## What solution would you like?

It'd be nice to have a way to implement relationships on immutable components, especially for one to one relationships.

I don't know if this is possible or not, someone who knows more about relationships will probably have to check.

```rust
#[derive(Component)]
#[component(immutable)]
#[relationship_target(relationship = RelationshipThingy)]
pub struct ThingyRelationship(Entity);

#[derive(Component)]
#[component(immutable)]
#[relationship(relationship_target = ThingyRelationship)]
pub struct RelationshipThingy(Entity);
```

## What alternative(s) have you considered?

The current solution of not doing this, however this does mean that relationships miss out on a lot of the benefits of immutable components.

The issue names no files, tests, or entry points. Start by reproducing the shown immutable relationship declarations and tracing the relationship and immutable-component validation; done would mean relationships can be declared on immutable components, especially the one-to-one case, with coverage for the supported behavior.

Written by the indexing model from the issue text.