build pants with its own rust backend!

[cosmicexplorer]

After we have a (more) fully-featured rust backend post-#8715 (possibly basing on top of #7260), we'll want to consider whether we can build pants with it! Considerations:

• How do we get a version of pants available to bootstrap the rust build? See pull down a previously-bootstrapped rust engine binary during development #8716.
• How do we orchestrate the "python generates scheduler.h => rust generates the scheduler binary, generating rust bindings for the scheduler with cbindgen => python uses cffi to connect to rust" sequence either (1) within cargo itself (2) via pants?
  • Is that even the right description?
  • If it's difficult to move over the logic from a lot of shell scripts at once, is it possible to e.g. integrate our custom shell scripts into a v2 process execution?
• Would this speed up CI or local development at all? Are there any cargo features such as pipelined builds we can turn on? See Implement the Cargo half of pipelined compilation (take 2) rust-lang/cargo#6883.

[Eric-Arellano]

@tdyas has started Rust work with #15093. Going to close this and let him do project management as he sees most appropriate