Support configurable CI/CD system notifications

[lorenyu]

This is an idea for a project to have the ability to configure CI/CD notifications, e.g. deploys, failed workflows, etc.

The idea is to be able to define two things in project-config:

1. channels – a list of channels for sending notifications to, including email channels and slack or Teams channels
2. topics – a map from topics to the channels that get sent notifications from those topics. e.g. topics can be levels like INFO, ERROR or things like DEPLOYS, INFRA, etc

When we'll have a custom notify action that can be called from workflows to trigger notifications. This can be used for workflow checks as well as things like deploys.

(discussed with @doshitan )