Feat: async driver for sqlalchemy-spanner

[giulia-dt]

This PR proposes an async implementation of the sqlalchemy driver for Spanner. This change allows Spanner to be used as a session database for ADK version >= 1.19.0.

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Fixes #15687 🦕

[gemini-code-assist]

Summary of Changes

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This pull request introduces asynchronous capabilities to the SQLAlchemy Spanner driver. By providing an async implementation, it allows the Spanner dialect to be used effectively with applications requiring non-blocking I/O, such as those built with ADK version 1.19.0 or newer. This enhancement significantly improves the driver's performance and responsiveness in concurrent environments.

Highlights

• Asynchronous Driver Implementation: A new asynchronous driver for sqlalchemy-spanner has been implemented, enabling non-blocking database operations.
• New Dialect Registration: The new asynchronous dialect, spanner+spanner_asyncio, has been registered, allowing users to specify async connections.
• Documentation Update: The README.rst file has been updated to include a new section on async support, providing usage instructions and an example.
• Unit Tests for Async Functionality: Dedicated unit tests have been added to validate the creation of async engines and the behavior of async connections.

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[gemini-code-assist]

Code Review

This pull request introduces asynchronous support for the SQLAlchemy Spanner driver, which is a great addition. The implementation correctly follows the standard SQLAlchemy pattern for async drivers by wrapping the synchronous DBAPI. The new documentation and unit tests are also well-implemented. I have a couple of minor suggestions to improve code clarity and documentation.