Isn't it part of the contact list nip or something?
Discussion
NIP02 has a variant but that form of petnames is (a) not supported in any clients that im aware of and (b) not preferred as
1) the namespace dot notation scheme is non deterministic
2) the petnames are public.
NIP32 labels is an option but thats not how that kind should be used per the nip.
NIP81 is what im using in Corny Chat and recommend to others