Is shuttle planning to support fork mode?

My application mutates global states (e.g., allocators, buffer pools) and may benefit from testing with separate processes. I'm wondering if it's possible for the shuttle to support fork mode. The first step could be making the [PortfolioRunner run in different processes](https://github.com/awslabs/shuttle/blob/main/src/runtime/runner.rs#L130).

Start at src/runtime/runner.rs around PortfolioRunner and determine how its execution could run in separate processes. Read the surrounding runtime code and establish what fork mode would need to support for applications with global state; done means the requested process-based shuttle mode is defined and working.

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