Add chat notification when deployment starts

Problem to solve

We already have chat notifications for successfull/canceled/failed deployments for slack, mattermost, etc... But not for starting deployments.

Notifications for starting deployments might be valuable, e.g. as warning that something is happening with production infrastructure right now.

Proposal

Add notification about starting deployments to chat integrations.

Note

It's highly coupled with #235474 (closed), so it will take less effort to work on these 2 issues simultaneously.