Allow the metdata route params to include callables:

- On entity creation, any callable is passed the object the entity
  is being created from.
- If the callable is a closure, and `bindTo()` is supported, the
  closure is bound to the object.
- The return value replaces the callable in the route param array
  before the array is passed to the `Link`.

Example with callable:
```php
    'route_params' => [
        'parent_id' => [$someService, 'getParentId']
    ]
```

Example with binding:
```php
    'route_params' => [
        'parent_id' => function(){
            return $this->getParent()->getId();
        }
    ]
```