Support authorizer to check allowed statements

[clue]

function myguard(int $action, ...$args)
{
    if ($action === SQLite3::READ) {
        list($table, $column) = $args;

        if ($table === 'users') {
            return SQLite3::OK;
        }

        return SQLite3::IGNORE;
    }

    return SQLite3::DENY;
}


$db = new Clue\React\Sqlite\SqliteClient(':memory:');

$db->on('open', function () use ($db) {
    $db->setAuthorizer('myguard');
});

$db->query('SELECT * FROM users')->then('var_dump');

See https://www.php.net/manual/en/sqlite3.setauthorizer.php (PHP 8+) and http://sqlite.org/c3ref/set_authorizer.html
Refs #47, #54, #58