Add a collection rule to validate collection data

[elie29]

In order to validate a collection of data, it would be helpful to have a CollectionRule that accepts a list of rules as follow:

       $rules = [
            ['email', EmailRule::class, EmailRule::REQUIRED => true],
            ['tags', CollectionRule::class, CollectionRule::RULES => [
                ['code', NumericRule::class, NumericRule::MAX => 80],
                ['slug', MatchRule::class, MatchRule::PATTERN => '/^[a-z]{1,5}$/i'],
            ]],
        ];

        $data = [
            'email' => 'elie29@gmail.com',
            'tags' => [
                ['code' => 12, 'slug' => 'one'],
                ['code' => 13, 'slug' => 'two'],
                ['code' => 15, 'slug' => 'three'],
            ],
        ];

        $validator = new Validator($data, $rules);

        assertThat($validator->validate(), is(true));

        $tags = $validator->getValidatedContext()['tags'];

        assertThat($tags, arrayWithSize(3));