$this->resolver
    ->setDefault('connections', static function (OptionsResolver $resolver) { // nested option
        $resolver
            ->setPrototype(true) // <- the new method
            ->setRequired('table')
            ->setDefaults(['user' => 'root', 'password' => null]);
    })
;

$this->resolver->resolve([
    'connections' => [
        'default' => [ // <- the array index "default" is optional and validation free
            'table' => 'default',
        ],
        'custom' => [
            'user' => 'foo',
            'password' => 'pa$$',
            'table' => 'symfony',
        ],
    ],
])

[
    'connections' => [
        'default' => [
            'user' => 'root',
            'password' => null,
            'table' => 'default',
        ],
        'custom' => [
            'user' => 'foo',
            'password' => 'pa$$',
            'table' => 'symfony',
        ],
    ],
]