[e2e] Slack integration for failed test runs

[BulatKha]

• to be aligned with DevOps for the required accesses to Slack
• most needed for regular regression suite execution status
• can be done for health check as well

[Haarolean]

1. We've used slack notifications for a helm workflow, which can be reused
2. Might be a good idea to duplicate these notifications into discord as well.