feat: Implement GeoSeries scalar operators

This commit implements 6 new GeoSeries scalar properties and methods:
- `is_empty`
- `geom_type`
- `is_ring`
- `is_simple`
- `is_valid`
- `union`

This change includes:
- Defining the new operations in `bigframes/operations/geo_ops.py`.
- Implementing the compilation logic for both Ibis and Polars backends.
- Adding the new properties and methods to the `GeoSeries` class.
- Adding unit tests for all new features.
- Adding system tests for all new features.

Author: google-labs-jules[bot]