The current implementation of ExtParamConfig in ERT stores parameter data as an xarray.Dataset, which is inconsistent with how scalar parameters (GenKwConfig) are handled using polars.DataFrame.
This should allow for better ensemble-level data operations and integration with the new storage and migration logic that assumes pl.DataFrame for scalar-like parameters.

Suggested approach:

class ExtParamConfig(GenKwConfig):
    type: Literal["everest_parameters"] = "everest_parameters"
    input_keys: list[str] = field(default_factory=list)