feat: ValidatedFrame when we need to return frames

[MartinBernstorff]

Makes it much easier to:

• Interpret the code statically
• Ensure column names match what is expected
• Document further expectations of the data
  • Column exists
  • Column type
  • Other checks which we can implement (is unique etc.)

Downsides:

• Makes the code more rigid

Alternatives:

• Pandera, but the API is weird, and doesn't support Polars

Implementation

ValidatedFrame is a dataclass with a post_init that runs all the validation. This means:

• Check that all attrs with col_name exist
• Check that all attrs with _schema match the schema