chore: add a new workflow file to send notifications upon new PRs, Issues, and comments

[amazreech]

Description of changes:

This PR creates a workflow file that gets triggered upon every Pull Request, Issue and Comments in the repository. The workflow file then sends relevant notification on the event.

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• Testing:*
• Tested on a personal repository, was able to successfully get notifications for the specified events.
• Test Link:* https://github.com/amazreech/simple-ecs-application/blob/main/.github/workflows/SlackNotification.yml

[amazreech]

Merged.