This is a Feature Proposal

Description

When a topic is specified with an arn, the notifications are ignored: https://github.com/ACloudGuru/serverless-plugin-aws-alerts/blob/master/src/index.js#L185

It would be great to have a way to add subscriptions to an existed topic when it's specified as an existing arn resource

For feature proposals:
Given an existing topic specified with arn
I would like to use the "notification" section of the plugin to add new subscriptions