Method to parse message boc

[mnill]

Please Please add a method that accepts message_boc: string and returns the message parsed into fields (or at least the body + body_hash part of it).

sign method that use for creating external messages returns SignedMessage {hash: string, expireAt: number, boc: string} but there is often a need to know the body and body_hash of this message and there is no way to obtain them.