feat: Add picosecond timestamp support

[google-labs-jules]

This change adds support for picosecond precision timestamps, aligning with recent changes in the BigQuery Java client.

• Modified src/types.d.ts to include timestampPrecision in ITableFieldSchema.
• Modified src/bigquery.ts to check for timestampPrecision === '12' in convertSchemaFieldValue. If detected, the value is returned as a string rather than being converted to a PreciseDate object (which only supports nanosecond precision), ensuring no data loss for picosecond values.

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PR created automatically by Jules for task 7439807773758684812 started by @danieljbruce

[google-labs-jules]

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