Support cloning RedisClient instance

[clue]

This changeset adds support for cloning an existing RedisClient instance using the clone keyword:

$original = new Clue\React\Redis\RedisClient($uri);
$redis = clone $original;

This can be especially useful if the original connection is used for a PubSub subscription or when using blocking commands or similar and you need a control connection that is not affected by any of this.

The implementation is relatively straight-forward. Major effort went into documenting this feature thoroughly and testing to avoid any unintended side effects. If you enjoy this work, consider supporting this project, for example by becoming a sponsor ❤️

Builds on top of #166, #163, #129 and others
Refs #118

[clue]

Rebased to resolve unrelated build error now that #173 is merged

[PaulRotmann]

Now that the tests run, the change LGTM.