Make `sandbox_image` configurable via the `initialize` event

[rbren]

What problem or use case are you trying to solve? Allow the user to specify which image to use in the sandbox, e.g. so it has all the right software installed

Describe the UX of the solution you'd like See https://github.com/OpenDevin/OpenDevin/issues/208

Do you have thoughts on the technical implementation? Would need to pull it out of the request here: https://github.com/OpenDevin/OpenDevin/blob/02a0367757b665171e9dff28fe434713382f2b92/opendevin/server/session.py#L116

and then plumb it through to the sandbox