feat: add FirestoreSessionService for serverless session persistence

[anmolg1997]

Summary

Add a FirestoreSessionService — a BaseSessionService implementation backed by Google Cloud Firestore, providing persistent, serverless session storage with zero database management.

This was originally submitted as google/adk-python#4439, where maintainer @rohityan directed it to this community repo.

Motivation

The ADK currently has:

• InMemorySessionService — dev-only, no persistence
• DatabaseSessionService — requires SQL infrastructure (PostgreSQL, MySQL, SQLite)
• VertexAiSessionService — requires a Reasoning Engine
• RedisSessionService (community) — requires a Redis instance

For GCP-native deployments (Cloud Run, Cloud Functions), Firestore offers a fully managed, serverless alternative with no infrastructure to provision.

Design

• Events stored in Firestore subcollections (avoids 1MB document size limit)
• Three-tier state management (app/user/session) matching ADK conventions
• Atomic state updates via Firestore transactions (addresses race conditions)
• Batch writes for bulk event deletion
• Server-side query limits for num_recent_events
• google-cloud-firestore as an optional dependency
• Async context manager support for clean client lifecycle

Implementation Plan

1. src/google/adk_community/sessions/firestore_session_service.py
2. tests/unittests/sessions/test_firestore_session_service.py
3. Update __init__.py exports and pyproject.toml optional dependencies

Ref: google/adk-python#3776