[chore] Create config-parseable Prefixes type

[tsmethurst]

i think in terms of getting v0.18.0 out, this looks good to go 👍 . but ideally instead of storing an extra ___Parsed field type we should instead just have a type that instead supports automatic unparsing otherwise it's just adding to our already cluttered config struct. i think wrapping the []netip.Prefix type to specifically support some kind of text unmarshaler interface will probably do the trick.

but before squerging this would you be able to make an issue to clean that up, and then once v0.18.0 is out get on it just so it doesn't get burried under our large tech-debt pile? :P

Originally posted by @NyaaaWhatsUpDoc in #3699 (comment)