RFE: provide a bluechi-online.target for synchronization on boot

[engelmi]

Please describe what you would like to see

BlueChi emits signals on the system D-Bus when nodes are dis-/connecting:
https://github.com/eclipse-bluechi/bluechi/blob/main/data/org.eclipse.bluechi.Node.xml#L224-L232

These can be used by state managers to determine when a node is ready to receive commands via BlueChi. However, currently it is not possible to write systemd services that run as soon as one or all bluechi-agents are connected on the controller machine (the same applies for the agent on its machine).

By defining a custom systemd target which only activates

• on the controller machine as soon as all agents are connected
• or on the agent machine as soon as it is connected to the controller

it is possible to use these targets as a synchronization point for other systemd units.

Please describe the solution you'd like

A POC for the controller machine can be seen in this gist:
https://gist.github.com/engelmi/ccd3bf8c0756adebcb73d431940d6c62

For this to work, we'd need to

• Implement small C program for listening on the node status (on controller machine)
• Implement small C program for listening on the agent status (on agent machine)
• Define .target and .service systemd units
• Add the new binary and systemd units to existing bluechi-controller and bluechi-agent packages in RPM
• Update documentation
• Implement integration tests