Add Nix logger activity bindings

[domenkozar]

Summary

• add a CXX-backed nix-bindings-logger crate
• expose process-global log messages and scoped build activities/build-log lines
• integrate the crate with native-input propagation and generated workspace docs

Why

Nix's C API does not expose logger emission. Code loaded into a Nix process—such as a store plugin—needs to create nix::Activity instances so build output appears in nix build -L without carrying an application-specific C++ shim.

Impact

Downstream plugins can reuse a small, process-safe Rust API for Nix logging while keeping application-specific plugin bindings separate.

Validation

• cargo fmt --all -- --check
• cargo test -p nix-bindings-logger with Nix 2.34
• parsed the changed Nix modules with nix-instantiate --parse

cc @roberth

[roberth]

Seems useful, but bypassing the upstream C API will be quite the can of worms.
Those C++ bits should really be done upstream.