[8.x] Introduce Conditional trait on validation rules

[inxilpro]

This introduces a new Conditional trait that adds the commonly-used ->when() and ->unless() helpers. I've limited this PR to adding these helpers to validation rules, although there's an opportunity to remove some duplication on the query builder, mailables, etc., if you want to expand the scope.

I find myself using when() more and more often, and usually miss it on objects that don't have it. One classic use case is in a FormRequest object:

class PageRequest extends FormRequest
{
  public function rules()
  {
    return [
      'slug' => [
        Rule::unique(Page::class, 'slug')
          ->when($this->isMethod('put'), function($rule) {
            $rule->ignoreModel($this->route('page'));
          }),
      ],
    ];
  }
}

Something like this lets me re-use the same ruleset for create and update operations. If I'm updating an existing page, I ignore that model from the unique rule. If I'm creating a new page, there's nothing to ignore.

This can be used to add stricter options to a password rule if the logged in user has privileged access, or dynamically changing image dimension requirements based on some dynamic input.

[driesvints]

Can you please use the same DocBlock style as we use in the rest of the framework? Also mixed can never be combined with something else. $this is already part of mixed. Thanks

[inxilpro]

Sure thing. I just copy-and-pasted this from the implementation in BuildsQueries, but I'll update it.

[inxilpro]

Although I think adding mixed|$this will provide significantly better code completion in IDEs—that's probably why it was there in the BuildsQueries DocBlock…