Introduce RustPasswordEngine, a BridgedEngine backed by the Rust logins component, used instead of the legacy JS PasswordEngine when the pref signon.storage.rust.active is set. Note: lastSync is not migrated from the legacy "<engine>.lastSync" pref into the Rust meta table, so the first sync after switching backends does a full re-download. This is a one-time, reset-like behavior. Differential Revision: https://phabricator.services.mozilla.com/D308535