Move current user_id to github_user_id and create a new user_id field

[chuckbutkus]

We are moving to using the Keycloak user ID as the OH user ID on the SAAS product.

This change enables us to do that.

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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:a2751b6-nikolaik   --name openhands-app-a2751b6   docker.all-hands.dev/all-hands-ai/openhands:a2751b6

[malhotra5]

Maybe also look into openhands/server/session/agent_session.py? There should be some references to github_user_id there