Add getTreeUsing

[nathanheffley]

This PR adds getTreeUsing which registers a callback that developers can use to customize what the tree looks like instead of automatically mapping it to a relationship.

This allows users to pass in an array in the proper format such as:

[
    [
        'name' => 'Parent Name',
        'value' => 'parent_value',
        'children' => [
            [
                'name' => 'Child Name',
                'value' => 'child_value',
                'children' => [], // optional if no children
            ],
           // etc...
        ],
    ],
    // etc...
]

The resulting value is an array made of the values of all selected options.

[CodeWithDennis]

Perhaps we can also allow public function getTreeUsing(Closure|array $value): static instead of only a closure?

Could you also add some README? @nathanheffley

[nathanheffley]

Could you also add some README?

Ah, I knew I was forgetting something! Yes, I can do both of those things 👍

[CodeWithDennis]

Thank you for adding this feature!