SSM Parameter to hold Slack channel

We would like to hold the Slack webhook URL in a secure SSM parameter so it isn't in source control.

Looks like we can pass a value into Lambda using that:

```typescript
const lambdaFunction = new lambda.Function(this, 'LogHandler', {
  environment: {
    SLACK_INCOMING_WEBHOOK_URL: ssmParameter.stringValue,
  },
});
```

Would you accept a PR for that if you agree?