Add Polars result type

[ptomecek]

Is your feature request related to a problem? Please describe.
Would like to be able to natively represent polars dataframes (eager or lazy) as a result type from a CallableModel

Describe the solution you'd like
Either a single result type for both types of frames, or two different result types (one for eager one for lazy). Optionally, we should be able to specify a schema that will get validated (as is done for the Arrow result type).

Describe alternatives you've considered
Purely using arrow tables works well for eager frames, but there are use cases where it is convenient to have callable models construct lazy frames instead to defer execution.