feat: add support for notifications on existing sns topics

[dougmoscrop]

What did you implement:

Closes #119

How did you implement it:

This adds new resources for notifications if there's no Topic resource being created.

How can we verify it:

I added some basic tests. I don't think documentation is necessary per se because this is actually what I think most people expected to happen based on the current documentation.

I tested it on my project and it worked. To test it, you just need to have something like:

topics: {
    alarm: {
        topic: {
            'Fn::ImportValue': 'monitoring-topic-arn',
        },
        // without this PR, this section just gets ignored and no subscriptions are created.
        notifications: [{
            protocol: 'lambda',
            endpoint: {
                'Fn::GetAtt': 'LambdaFunction.Arn',
            },
        }],
    },
},

Todos:

[x] Write tests
[x] Write documentation
[x] Fix linting errors
[x] Provide verification config/commands/resources