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.
Edited by Vladimir Shushlin