Implement proto2/proto3 with editions

This change migrates proto2 and proto3 to real editions, without impacting users at all.  While we already had feature defaults set, we weren't really using them outside of editions.  This change introduces InferLegacyProtoFeatures, which infers features from descriptors.  Any language that wants to follow this rollout pattern will need to implement similar transformations.

PiperOrigin-RevId: 572593956