The `BarrierSpec` is missing a `discardChannel` option

[artembilan]

Just expose a BarrierSpec.discardChannel() options according to the BarrierMessageHandler setters:

	/**
	 * Set the name of the channel to which late arriving trigger messages are sent.
	 * @param discardChannelName the discard channel.
	 * @since 5.0
	 */
	public void setDiscardChannelName(String discardChannelName) {
		this.discardChannelName = discardChannelName;
	}

	/**
	 * Set the channel to which late arriving trigger messages are sent.
	 * @param discardChannel the discard channel.
	 * @since 5.0
	 */
	public void setDiscardChannel(MessageChannel discardChannel) {
		this.discardChannel = discardChannel;
	}

Related SO thread: https://stackoverflow.com/questions/79101282/spring-integration-do-something-when-timeout-occure