feat: support load_table and list_rows with picosecond timestamp

[Linchin]

• Support picosecond in Client.load_table_from_file()
• Support picosecond in Client.list_rows()

[gemini-code-assist]

Summary of Changes

Hello @Linchin, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request integrates a new capability into the BigQuery client library, allowing for precise control over timestamp data during table load operations. By introducing the timestamp_target_precision property, users can now define the subsecond precision for TIMESTAMP columns, including support for picoseconds, ensuring that time-sensitive data is accurately handled and stored when using autodetection, especially for CSV data sources.

Highlights

• New Feature: Timestamp Target Precision: Introduced a new timestamp_target_precision property to the LoadJob class, allowing users to specify the desired precision for TIMESTAMP types during BigQuery table loading with autodetection.
• Picosecond Timestamp Support: This enhancement enables the handling of picosecond-level timestamp data (up to 12 digits of subseconds), providing more granular control over timestamp interpretation, particularly for CSV-formatted data.
• Comprehensive Unit Tests: Added a suite of unit tests to test_load_config.py to ensure the correct functionality of the timestamp_target_precision property, covering its getter, setter, and proper serialization/deserialization with API representations.

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[gemini-code-assist]

Code Review

This pull request adds support for timestamp_target_precision in LoadJobConfig, which allows specifying target precisions for TIMESTAMP types during data loading with autodetection. The implementation looks good and is well-tested.

For future improvement and API consistency, consider exposing this new property on the LoadJob class as well, similar to how other LoadJobConfig properties are exposed. This would involve adding a corresponding read-only property to LoadJob that delegates to self.configuration.timestamp_target_precision.

I've also left a minor comment about a style issue in the docstring.