rust: Add missing Rust allow path modules.

Adds rust_benchmark and rust_fuzz to the list of Rust module types which
need to be in the allowed paths, and a comment about rust_bindgen and
rust_protobuf.

This also adds a downstream allow path list which helps prevent merge
conflicts if downstream wants to extend the list of allowed paths.

Bug: 191507775
Test: Paths are checked for the newly added module types.
Test: Paths in DownstreamRustAllowedPaths allow Rust modules.
Change-Id: Ida80c33a815d47ffdfb1f648125d71316a2a9d8a