Review Mozilla 1914143: [meta] Implement a Rust based SearchEngineSelector

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1914143

Upstream is experimenting with a search engine selector written in Rust, for sharing more code between desktop and mobile platforms.

We already customize our search engines, but with this we will have to rework such patches once again.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information