- Closes #3159.
- Moves `protocol.Envelope` to `pkg/types/messaging`.
  - The protocol package is a monster and I want to avoid adding more, hence messaging being its own package.
  - To support message types other than user transaction and user signature, an envelope needs to contain arbitrary messages.
  - Thus Envelope has Message and UserTransaction has Transaction, leading to a circular dependency between the packages. The simplest way to resolve this (and not add to the protocol package bloat) is moving Envelope to messaging.
- Eliminates NormalizeLegacy and LegacyMessage, instead always normalizing an envelope to UserTransaction and UserSignature messages, then leaving it up to the executor to reconstruct deliveries from those messages.

Changelog: refactor