feat(protobuf): improve ergonomics by combining decoding and verification

[Mirko-von-Leipzig]

Users of the miden-protobuf types have to do a repetitive dance of:

message
  .decode_fields()
  .map_err(..)
  .verify()
  .map_err(..)?

This PR adds an extension trait DecodeMessageExt which provides the decode_and_verify(), decode_and_verify_with(), and decode_and_build_unchecked() methods via blanket impls where appropriate.

This is an ergonomic improvement.

[kkovaacs]

Looks good to me!

[zeapoz]

Looks great! Left a tiny nit about the changelog entry.