(aws-chatbot): Add notification topics to an existing SlackChannelConfiguration construct

I'm looking for the ability to add SNS notificaiton topics to a `SlackChannelConfiguration` object outside of its constructor.


### Use Case

Define a channel configuration in one stack and add some SNS topics to it in another



### Proposed Solution

This would probably be a `addNotificationTopics` class method.



This is a :rocket: Feature Request
