arti: BridgesConfig is evidently not too high up

/// Configuration for bridges and pluggable transports

//

// TODO pt-client: This type is too high up the stack:

// It is likely that this type will want to move much lower down in the crate

// stack so that (eg) guardmgr can actually do something useful with the `BridgeList`

// and resolve the situation with `enabled` and so on.

//

// Possibly guardmgr will want to take an `Arc<dyn AsRef<BridgesConfig>>` or something;

// that would enable passing it an "extract" from the config without actually copying it.

//

// We leave this as an empty struct even when bridge support is disabled,

// as otherwise the default config file would generate an unknown section warning.

#[derive(Debug, Clone, Builder, Eq, PartialEq)]