feat(bigquery): add column data policies to SchemaField

[AliAbouelazm]

Adds data_policies to SchemaField so callers can read and set column data policy names without using _properties. Names are serialized as dataPolicies entries, and equality and hashing account for them regardless of order.

The tests cover unset, null, empty, multiple and nested policies, API round trips, repr, and schemas that use policy tags on other columns. The usage guide explains the API representation and the limits around removing the last policy and dataPolicyList precedence.

Validation on Python 3.12: 2,859 unit tests passed, 2 skipped. Black, flake8, mypy and the documentation build passed. The test process used default OpenTelemetry sampling because the inherited setting caused the same seven failures on unmodified main.

Fixes #17617.

[gemini-code-assist]

Code Review

This pull request introduces support for column-level data policies in BigQuery schemas by adding a data_policies property to SchemaField, along with documentation and comprehensive unit tests. The review feedback suggests two key improvements: adding a type check to prevent strings or bytes from being silently iterated character-by-character when passed to data_policies, and replacing fragile index slicing ([:-1]) in the __repr__ method with dynamic filtering based on the named tuple's fields to ensure future-proof code.