adapters/application: Pass `User-Agent` header to backend in fastboot mode
adapters/application: Pass `User-Agent` header to backend in fastboot mode

The backend was complaining about only accepting requests with a `User-Agent` header, so let's give it such a header ;)

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io