code: async function signTransaction (tx) {
    const networkId = this.getNetworkId()
    const rlpBinaryTx = Crypto.decodeBase64Check(Crypto.assertedType(tx, 'tx'))
    // Prepend `NETWORK_ID` to begin of data binary
    const txWithNetworkId = Buffer.concat([Buffer.from(networkId), rlpBinaryTx])
  
    const signatures = [await this.sign(txWithNetworkId)]
    return buildTx({ encodedTx: rlpBinaryTx, signatures }, TX_TYPE.signed).tx
  }
docs: Sign encoded transaction
  @instance
  @category async
  @rtype (tx: String) => tx: Promise[String], throws: Error
  @param {String} tx - Transaction to sign
  @return {String} Signed transaction