Support rustc incremental compilation via persistent workers

[anguslees]

Summary

• add protobuf persistent-worker support directly to process_wrapper
• add an opt-in --@rules_rust//rust/settings:experimental_persistent_worker=true setting
• reuse request-namespaced rustc incremental state for non-optimized builds
• keep process-wrapper flags in worker startup arguments and materialize each flat WorkRequest as one rustc param file
• preserve nested rustc response files and process-wrapper substitutions with an internal response-file marker
• preserve one-shot execution for remote/non-worker executors and the rare opaque Args extension case

Related to #412. This is a worker-local alternative to the Bazel-managed incremental artifacts proposed in #3925.

Test plan

• bazel test //util/process_wrapper:process_wrapper_test --@rules_rust//rust/settings:experimental_persistent_worker=true
• bazel test //test/process_wrapper/... //test/unit/pipelined_compilation:pipelined_compilation_test_suite //test/unit/location_expansion:location_expansion_test_suite
• worker and forced one-shot builds of //test/unit/location_expansion:mylibrary with nested response files
• worker build with a response file supplied through extra_rustc_flag
• bazel test --config=rustfmt //util/process_wrapper:process_wrapper_test
• buildifier -mode=check rust/private/rust.bzl rust/private/rustc.bzl rust/settings/BUILD.bazel rust/settings/settings.bzl

AI assistance

Kilo Code assisted with rebasing the original implementation onto current main, compatibility review, and test iteration. I reviewed and own the resulting changes.

[anguslees]

If you would rather I switch to using a protobuf library (and add more deps to the process wrapper), I can do that instead.