Tracking: Making Narwhals extensible

[dangotbanned]

Add this as a parent issue if you find something that breaks/adds difficulty to extending narwhals

[FBruzzesi]

Not sure if this is the best issue where to leave this thought.

We might have overlooked at #2001 which has great potential and points towards a likable pattern.

In a similar way, we might need to move Implementation class from an enum to a registry pattern (related to #2786), in fact there is no way to extend (inject a new value) an enum without inheriting from it, but that would be too late for extensions