feat(firestore): add BSONBinary support

[ohmayr]

This PR adds support for BSONBinary in the Firestore Python SDK. BSONBinary` allows database applications to store subtyped binary data payloads (such as UUIDs, MD5 hashes, or custom binary subtypes) in Firestore documents.

• BSONBinary Container Class: Implements BSONBinary(_BSONType) in google.cloud.firestore_v1.bson, wrapping binary payloads (bytes or bytearray) and a 1-byte BSON subtype tag ($0 \le \text{subtype} \le 255$, defaulting to 0).
• Spec-Compliant Wire Serialization (_to_map_value):
  • subtype == 0 (generic binary): Serializes directly as raw bytes (bytes_value).
  • subtype != 0 (subtyped binary): Serializes as {"__binary__": bytes([subtype]) + data} (map_value).
• Defensive Validation:
  • Raises TypeError for non-binary payloads or non-integer / boolean subtype tags.
  • Raises ValueError if subtype is outside the 1-byte range ($0 \le \text{subtype} \le 255$).
• SDK Exports & Post-Processing: Exported BSONBinary in google.cloud.firestore and google.cloud.firestore_v1, and updated .librarian/generator-input/client-post-processing/firestore-integration.yaml.
• Tests: Added 14 atomic unit tests in tests/unit/v1/test_bson.py and verified live sync and async write operations against Firestore Enterprise DB (enterprise-db-native-2).

Fixes b/562182261 🦕

[gemini-code-assist]

Code Review

This pull request introduces the BSONBinary class to represent BSON binary data containers with subtypes for Firestore. It includes integration with the client post-processing configuration, exports the class in the package's public interfaces, and adds comprehensive unit and system tests to verify its behavior and serialization. I have no feedback to provide as there are no review comments.