Add support for Modal

[azliu0]

What does this implement/fix? Explain your changes.

Adds support for running the evaluation harness on Modal Labs.

Adding a --modal flag to the normal run command will run the instances remotely rather than locally. The current images resolves ~290 tasks on Lite.

Benefits of running this way:

• users don't have to set up their own infrastructure, everything runs in the cloud ☁️
• everything runs in parallel, so building and running larger numbers of instances is effortless. Lite builds and runs from scratch in 20-30min and should work out of the box
• Modal's image caching is built-in and follows essentially the same layered abstraction as the images already in the repo

The only downside for now is that the images are slightly less reliable/well tested than the local images.

[wenting-zhao]

I am curious if Modal has the plan to fix the remaining 10/300?