Example usage of httpx

[tofarr]

End-user friendly description of the problem this fixes or functionality that this introduces

This PR updates the GitHub API integration to use httpx instead of requests for making HTTP requests. This change improves the async support in our GitHub routes, making them more efficient and performant.

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

Improved performance of GitHub API integration by using modern async HTTP client.

---

Give a summary of what the PR does, explaining any non-trivial design decisions

This PR replaces the synchronous requests library with the asynchronous httpx library in our GitHub API integration routes. Key changes include:

1. Removed the need for call_sync_from_async by using native async HTTP calls
2. Better resource management using async context managers
3. More efficient handling of HTTP responses
4. Maintained existing functionality while improving performance

---

Link of any specific issues this addresses

N/A

---

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:5c644ce-nikolaik   --name openhands-app-5c644ce   docker.all-hands.dev/all-hands-ai/openhands:5c644ce

[rbren]

constructing a JSONResponse here is odd, but it looks like we were already doing it...