feat: add support for Firestore data bundles

This commit introduces the `BundleBuilder` class, which allows for the creation of Firestore data bundles. Bundles can include document snapshots and named query snapshots.

The `BundleBuilder` serializes these elements into a length-prefixed JSON format that can be served to clients for pre-loading data, enabling faster initial load times and offline access.

The implementation includes:
- `BundleBuilder` to add documents and queries.
- `build()` method to generate the final `Uint8List` bundle.
- Internal models for bundle elements (`BundleMetadata`, `BundledDocumentMetadata`, `NamedQuery`, etc.).
- Helper methods for JSON serialization of Firestore types.
- New unit and integration tests for the bundling functionality.

Author: demolaf

packages/googleapis_firestore/lib/googleapis_firestore.dart

@@ -49,4 +49,5 @@ export 'src/firestore.dart'
         DocumentChangeType,
         Precondition,
         TransactionHandler,
-        SetOptions;
+        SetOptions,
+        BundleBuilder;