Expose "append signature" utility

[0x009922]

Description

Appending signature to an existing SignedTransaction is a common operation for multisignature workflows.

The API should look like this:

import { Signer, appendTransactionSignature } from '@iroha2/client'
import { datamodel } from '@iroha2/data-model'

/** @type {datamodel.VersionedSignedTransaction} */
let tx

tx = appendTransactionSignature(tx, keypair)

// or through `Signer`
tx = new Signer(keypair).appendTransactionSignature(tx)