feat: Implement GeoSeries scalar operators

[tswast]

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.

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• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
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• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕