[Experimental] Trying async downloads

[googlyrahman]

These changes are prototype for async downloads to compare the performance of gcsfs async with SDK async. These changes should not be merged.

[gemini-code-assist]

Summary of Changes

Hello @googlyrahman, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request serves as an experimental prototype to evaluate the performance of asynchronous downloads within the Google Cloud Storage SDK, specifically comparing it against gcsfs async. It establishes foundational async client components and a flexible download mechanism that supports both traditional sequential and advanced sliced (parallel) downloading for large files. The author has indicated that these changes are for testing purposes and are not intended for merging.

Highlights

• Async Client Infrastructure: Introduces Client and ClientWithProjectAsync classes in _helpers.py to manage asynchronous credentials and HTTP sessions using aiohttp_requests.
• Asynchronous Download Client: Adds AsyncClient in async_client.py with a download_to_file method, enabling both sequential and experimental sliced (parallel) downloads for large objects.
• Sliced Download Logic: Implements a _check_if_sliced_download_is_eligible method to determine if a file (over 1GB and without checksum validation) can be downloaded in parallel chunks.
• Async Download Mechanism: Defines DownloadAsync in download.py to handle the actual asynchronous streaming of data, including partial content handling and optional checksum validation.

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[gemini-code-assist]

Code Review

This pull request introduces an experimental implementation for asynchronous downloads, including a parallel sliced download strategy. The code is a good starting point for a prototype. My review focuses on several areas for improvement, particularly around the use of synchronous I/O within async functions, which can block the event loop and negate the benefits of asyncio. I've also pointed out opportunities to improve code clarity by removing dead code and replacing magic numbers with constants. There are also some potential bugs related to status code handling and hardcoded values that should be addressed.