feat: support local development using Buck2

[bolinfest]

This is an experiment to get the Rust portion of the Codex repo setup to build with Buck2.

The motivation is to get build time benefits, both for local/incremental builds as well as in CI, though of note:

• For reference, general advantages of Buck2: https://buck2.build/docs/getting_started/what_is_buck2/#why-use-buck2-key-advantages
• Locally, Buck2 runs a daemon, which helps speed up incremental builds.
• Using Buck2 means we can ultimately leverage distributed builds
• Because Buck2 is stricter on running builds and tests in an "isolated" environment, it holds us to a higher standard to ensure builds are hermetic
• Buck2 has a complementary project, Reindeer, which is used to create BUCK build files for existing crates: https://github.com/facebookincubator/reindeer (I am not aware of a comparable offering for Bazel)

Unfortunately, buck2 test does not currently cache test results as Bazel does:

facebook/buck2#183

This is pretty important, as we want incremental invocations of:

./scripts/buck2 test //codex-rs/a

to be as fast as possible, so I'll follow up on this.