In
  tpo/core/arti!462 (comment 2797697)
we decided not to do this.

However, having looked again at the way the FallbackList works, I
think there is a lot of value in making these two things (and anything
else like them[1]) as similar as possible.

[1] At least PreemptiveCircuitConfig.initial_predicted_ports and
NetworkConfig.authorities need the same treatment, and perhaps also
GuardUsage.restrictions (although there is no
GuardRestrictionBuilder).

In the irc discussion I imagined `LogfilesConfigBuilder` as opposed to
`LogfileConfigBuilder` (differing only in the `s`) which would be bad,
but we can use `List` instead.

We do *not* need to abstract away the validated version of the config.
Providing a type alias helps the derive_builder sub_builder DTRT
without needing special overrides.

I have split this commit so that we can drop it, if we conclude it's
not wanted.