fix(python): Include pl. qualifier for inner dtypes in to_init_repr()

[datenzauberai]

This is a minimal fix for #15802 by adding a method that prefixes representations, which will be called "recursively" for nested types. Idea:

• Series.to_init_repr will call the new method _prefixed_repr with the prefix="pl."
• for normal DataTypes _prefixed_repr will use __repr__ and prefix it with the given prefix => no further changes necessary
• for NestedTypes the flow of control is reversed __repr__ will call _prefixed_repr with no prefix. _prefixed_repr will take care of passing the prefix for the inner dtype representation

[stinodego]

Thanks for working on this. I don't like the idea of adding this as a method on the class though as it's a pretty niche thing. I'd prefer it to exist as a utility function.