The universal ID used to represent the Persona outside its Backend.
This is opaque, and should only be parsed by clients using Persona.split_uid.
This is the canonical way to refer to any Persona. It is guaranteed to be unique within the Persona's PersonaStore.
A Persona's UID is immutable over the life of the Persona in the backing store, so a given UID is guaranteed to refer to the same Persona each time libfolks is used, until the Persona is permanently removed from its backing store.