[toolchains] Add opt-in nightly rustc-dev toolchains

[andyscott]

Summary

• add opt-in rustc-dev support for generated nightly Rust toolchains
• keep the default toolchain path unchanged, with separate dev-enabled variants selected only when requested
• add regression coverage for dev-enabled versus ordinary nightly toolchain inputs

Motivation

Generated rules_rs toolchains currently cannot support nightly crates that depend on rustc_private, because those crates need the rustc-dev component in the compiler sysroot.

This adds that capability as an explicit nightly-only opt-in so downstream users can support rustc_private workloads without making ordinary toolchains larger or changing default selection behavior.

Details

• add include_rustc_dev = True to the toolchain module extension
• fetch rustc-dev separately from ordinary rustc
• materialize merged rustc_with_dev sysroots only for dev-enabled toolchains
• expose parallel *_with_dev toolchains behind @rules_rs//rs/toolchains/rustc_dev:enabled
• document the required nightly/dev flags
• add analysis tests covering presence and absence of compiler .rlib inputs

Validation

• bazel build //rs/toolchains:module_extension //rs/private:rustc_dev_repository //rs/private:rustc_with_dev_repository