FFI Transaction: add signing_digest_hex() and bcs_serialize_base64()

[Thoralf-M]

To make it easier in bindings, currently many examples do this

        print("Signing Digest:", hex_encode(txn.signing_digest()))
        print("Txn Bytes:", base64_encode(txn.bcs_serialize()))

[thibault-martinez]

Closed by #300