Add new public callAsync() method

[clue]

This changeset adds a new public callAsync() method to the RedisClient. Some static analysis tools may not understand the magic __call() method, so you may now use the new callAsync() method as a more explicit alternative.

// old (still supported)
$redis->get('name');

// new
$redis->callAsync('GET', 'name');

On top of this, this is also a prerequisite for adding a blocking API using Fibers on PHP 8.1+ in a follow-up PR.

Builds on top of #140, #141, #129, #127 and others
Refs #118