Mark a configuration as having passed validation before using it

[iffyio]

Some config fields have validation rules which we check on startup. For example, that a list of endpoints must be non-empty. However, when we later try to fetch that retrieve that list from the config we don't have any way to guarantee that the list is in fact non-empty.
This leads to either re-validating the list or trusting that validation did in-fact take place (and possibly panicking if the assumption is wrong).

Instead, we can have a ValidatedConfig type that is returned by the validation logic guaranteeing that validation took place. e.g Input config has the format

struct Config {
  endpoints: Vec<EndPoint>, // Serialized from disk, may or may not be empty.
}

while the returned, validated config has the same format but with a new type

struct ValidatedConfig {
  endpoints: Endpoints, // Endpoints is a type that guarantees that the internal list is non-empty.
}