feat: support way of providing clients through a callable to simplify runtime modification of clients dict

[MelchiorKokernoot]

During development on our software using this package, we encountered the need to alter the clients dict on runtime. Altough possible to do this through external (outside of the package) means, we reckoned this might be a feature that could be easy to implement in the package.

I have written the clients dict parameter to an internal _clients dict, which is now accessed through an annotated prop. When accessing the prop a check is done if the actual value is callable and is called instead of just directly accessing the dict.

Tests have been added