code: def sign(tx_template)
        return tx_template if @xpubs_by_signer.empty?
  
        @xpubs_by_signer.each do |signer_conn, xpubs|
          tx_template = signer_conn.singleton_batch_request(
            '/sign-transaction',
            transactions: [tx_template],
            xpubs: xpubs,
          ) { |item| Transaction::Template.new(item) }
        end
  
        tx_template
      end
docs: Sign a single transaction
   @param [Hash] tx_template	A single transaction template.