Switched to library mode for generating bindings. Updated our strategy for UniFFI components/fixtures. There's now a crate for components and a crate for fixtures. These crates are a dependency of gkrust-shared and also built as standalone libraries which UniFFI uses to generate the bindings. One upshot of this is that we no longer need the `crate_name` or `udl_file` config values. library-mode can figure out the udl paths automatically. Differential Revision: https://phabricator.services.mozilla.com/D221824