[OptionsResolver] Support nested options #4833

It is currently not possible to configure nested options. For example:

$resolver->setDefaults(array(
    'db' => array(
        'dsn' => null,
        'user' => 'root,
        'password' => '',
        'port': 3306,
    ),
));

It is not possible to resolve this configuration in the following way:

$resolver->resolve(array(
    'db' => array(
        'dsn' => ...
    ),
));

because now the other default values under "db" will be dropped. Also, it is not possible to specify allowed values or types for the nested options.