Support `__aenter__`/`__aexit__` for experimental-async coroutines

Summary

We are using PyO3's experimental async support for coroutines, but currently, these coroutine objects do not support the async context manager protocol.

Because of this, we have to await the response first before we can use it in an async with block. The current code looks like this:

import asyncio
import rnet
from rnet import Response

async def main():
    # Currently, we must await the coroutine first separately
    resp: Response = await rnet.get("https://httpbin.io/stream/20")
    async with resp:
        async with resp.stream() as streamer:
            async for chunk in streamer:
                print(chunk)
                await asyncio.sleep(0.1)

if __name__ == "__main__":
    asyncio.run(main())

Expected Behavior

The expected usage should be much cleaner, allowing us to use async with directly on the awaited coroutine:

import asyncio
import rnet

async def main():
    # The goal is to support this one-liner pattern
    async with rnet.get("https://httpbin.io/stream/20") as resp:
        async with resp.stream() as streamer:
            async for chunk in streamer:
                print(chunk)
                await asyncio.sleep(0.1)

if __name__ == "__main__":
    asyncio.run(main())

No files or tests are identified in the issue. Start by locating PyO3's experimental-async coroutine implementation and its existing protocol support, then reproduce the documented examples; done means the coroutine returned by rnet.get can be used directly in an async with block while preserving the response streaming behavior.