Provide an "easy" way to connect backend service to frontend notification systems

[sanderegg]

The idea is to leverage our RabbitMQ messaging system to this end.

First a update/refactoring step is necessary:

• https://github.com/ITISFoundation/osparc-simcore/pull/3496
• https://github.com/ITISFoundation/osparc-simcore/pull/3492
• https://github.com/ITISFoundation/osparc-simcore/pull/3502

Next is to use the common rabbitmq client to send messages from:

• [x] autoscaling application to provide user feedback about cluster scaling
• [x] dynamic-sidecar powered services to provide user feedback on services states, pull/push progress
• [x] allow for disconnecting users for maintenance purpose

[sanderegg]

all the points were completed