TROVE-2023-002 bridges.enabled=true can be accepted if bridge support compiled out

Steps to reproduce

cargo build --no-default-features --features=native-tls,tokio -p arti

Make ~/.config/arti/arti.toml containing

[bridges]
enabled = true

(but no actual bridges)

Run target/debug/arti proxy

Expected behaviour

Arti refuses to start, saying:

target/debug/arti: error: read configuration: Config semantically incorrect: Fields ["bridges.enabled", "bridges.bridges"] are inconsistent: bridges enabled=true, but no bridges defined

Actual behaviour

Arti runs, and connects to guards directly.