Conditional relationships

[lindyhopchris]

At the moment the resource objects support conditional attributes in the attributes and meta values. We should also add support for conditional relationships.

To do this, we need to support (in the encoder) having relations wrapped in either a conditional attribute or conditional attributes object. So the syntax on the resource would be:

public function relationships($request): iterable
{
    return [
        $this->relation('author'),
        $this->when($request->user()->isAdmin(), $this->relation('publishedBy')),
        $this->mergeWhen($request->user()->isOwner(), [
            $this->relation('foo'),
            $this->relation('bar'),
        ]),
    ];
}

Probably we should rename the ConditionalAttr and ConditionalAttrs classes to ConditionalField and ConditionalFields considering we're now not just using them in attributes.