text: Title: [ENH] `polars` schema checks - address performance warnings
  Body: The current schema checks for lazy `polars` based data types raise performance warnings, e.g.,
  
  ```
  sktime/datatypes/tests/test_check.py::test_check_metadata_inference[Table-polars_lazy_table-fixture:1]
    /home/runner/work/sktime/sktime/sktime/datatypes/_adapter/polars.py:234: PerformanceWarning: Determining the width of a LazyFrame requires resolving its schema, which is a potentially expensive operation. Use `LazyFrame.collect_schema().len()` to get the width without this warning.
      metadata["n_features"] = obj.width - len(index_cols)
  ```
  
  These should be addressed.
  
  The tests to execute to check whether these warnings persist are those in the `datatypes` module - these are automatically executed for a change in the impacted file, on remote CI.