Replace `Vec<ValidatedThing>` in configs with ThingListBuilder

Following on from !462 (merged), the following fields need the same treatment as was done there to FallbackList and LogfileConfig:

• PreemptiveCircuitConfig.initial_predicted_ports (maybe)
• NetworkConfig.authorities (surely)
• GuardUsage.restrictions (although there is no GuardRestrictionBuilder?)

The impls for FallbackListBuilder and LogfileListConfigBuilder are strikingly similar. It would be good to combine them, probably using a declarative macro.