feat: update demo to run child devices using tedge-agent

[reubenmiller]

Promote the fact that the tedge-agent can also be run on child devices so that users don't need to develop there own custom agents to manage child devices (which run a Linux OS or as a container)

The main demo now has the following setup:

• main device (with systemd)
• child01 - tedge-agent deployed in a multi-process (s6-overlay) container
• child02 - tedge-agent deployed in a systemd based container

An example firmware_update workflow has also been added to the main and child devices however it won't be visible until thin-edge.io 1.3.1 is release (due to https://github.com/thin-edge/thin-edge.io/issues/3145)

[github-actions[bot]]

Robot Results

:white_check_mark: Passed	:x: Failed	:next_track_button: Skipped	Total	Pass %	:stopwatch: Duration
20	0	2	20	100	2m39.8s