Feature: Add support for conditional slack notifications

Use case

It would be great to add built-in support to conditionally send slack notifications. One use case is, send notification only for specific deploys Prod / QA and not dev or feature branches.

Suggested

custom:
  notification:
    deployer: /* Default deployer name*/
    slack:
      enabled: {{self:provider.stage}}

enabled flag is true we perform notification.

Since sendNotification is the sink of all promises might as well put it there. or skip the entire promise chaining.

Also thanks for this plugin. 🙏

Start in index.js at the sendNotification sink linked by the issue, then trace how the Slack notification configuration is read. Confirm how the suggested enabled value should control notifications for different Serverless stages, and verify that enabled notifications still send while disabled ones do not.