Refactor: add RuntimeManager class

[rbren]

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

This should be a pretty clean no-op. But I really want to expand this so that there are separate implementations for each runtime class. Right now the runtime classes themselves are doing a lot of extra lifting for managing runtimes in an external environment (docker, runtime-api, 3p providers)

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Link of any specific issues this addresses

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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:9a6084c-nikolaik   --name openhands-app-9a6084c   docker.all-hands.dev/all-hands-ai/openhands:9a6084c