[8.x] Add exclude validation rule

[bastien-phi]

This PR adds an exclude validation rule which exclude the field from validated data.

This validation

public function rules()
{
    return [
        'role' => ['required', in:user,guest'],
        'email' => [
            'required_if:role,user',
            'exclude_unless:role:user',
            'string',
            'email,
         ],
     ];
}

is quite easy but we need to take a close look to email validation.

We can take advantage of Rule::when for email field to make the validation quite easier to read :

public function rules()
{
    return [
        'role' => ['required', 'in:user,guest'],
        'email' => Rule::when(
            function ($data) => $data->get('role') === 'user',
            ['required', 'string', 'email'],
            'exclude'
       ),
   ];
}

This way, the validation is easier to read.

[bastien-phi]

Thanks !