PYTHON-4493 - Use asyncio protocols instead of sockets for network IO

[NoahStapp]

Opening this now to begin the review process, as these changes are significant and especially critical to the driver's behavior.

Remaining TODOs:

• [ ] Ensure exhaust cursors fetch only batch_size messages at a time, rather than all at once.
• [ ] Verify our entire EG test suite passes
• [ ] Async performance benchmarks (PYTHON-5144)

A first review pass should focus on the structure of the code, especially PyMongoProtocol and async_receive_message.

PyMongoProtocol reads complete MongoDB wire protocol messages instead of raw bytes. This significantly increases the complexity of its read and buffer_updated methods, but allows us to maximize performance and encapsulate the wire protocol within.

For additional context on the Python Transport/Protocol API, see the official docs and this discuss thread that forms the backbone of our approach here.