Add HTTP FileStore implementation

[tofarr]

• [ ] This change is worth documenting at https://docs.all-hands.dev/
• [x] Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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

This PR adds a new HTTP-based FileStore implementation that allows OpenHands to store and retrieve files using a remote HTTP server. This enables more flexible storage options beyond local files, in-memory storage, and S3.

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Summarize what the PR does, explaining any non-trivial design decisions.

This PR adds:

1. A new HTTPFileStore class in openhands/storage/http.py that implements the FileStore interface using HTTP requests
2. Unit tests for the HTTPFileStore class

The implementation supports:

• Simple session api key authentication out of the box
• Custom headers using a custom httpx client.
• Error handling with appropriate exceptions

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Link of any specific issues this addresses:

N/A

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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:107625e-nikolaik   --name openhands-app-107625e   docker.all-hands.dev/all-hands-ai/openhands:107625e