txnbuild/transaction: add a way of doing external signing of transactions without transaction.Sign(..)

[Pownieh]

What problem does your feature solve?

This allows users of the stellar-go library to implement their own signing algorithm. This is beneficial for anyone wanting to sign without having direct access to the private key (and thus cannot produce the required KeyPair), such as when using a cold wallet or using a secret shared private key.

What would you like to see?

The signing algorithm requires hashing the TransactionEnvelope, which is currently a private property (envelope) of the Transaction type. Once this envelope has been hashed and signed, this has to be set in the transaction, something that also cannot be done (without using clone(...) of txnbuild/transaction). Therefore, adding either a way of getting the envelope directly (like done here), or adding a specific way of getting the hash of the envelope and then afterwards setting the signatures, would allow someone to implement their own signing protocol.

What alternatives are there?

Alternatives to the two approaches described above include injecting once own signing function directly and likely other options.