[8.x] Add strict comparison to distinct validation rule

[gilbertorussi]

Adds the ability to add the parameter strict to the distinct validation rule.

$v = new Validator($trans, ['foo' => ['0100', '100']], ['foo.*' => 'distinct'], ['foo.*.distinct' => 'There is a duplication!']);
$this->assertFalse($v->passes());
$v->messages()->setFormat(':message');
$this->assertSame('There is a duplication!', $v->messages()->first('foo.0'));
$this->assertSame('There is a duplication!', $v->messages()->first('foo.1'));

$v = new Validator($trans, ['foo' => ['0100', '100']], ['foo.*' => 'distinct:strict']);
$this->assertTrue($v->passes());

The default behaviour was not changed.