Add sd_notify to report agent status to systemd

[sbienkow-ninja]

Is your feature request related to a problem? Please describe. Currently, k8s-less installs of aperture-agent lack healthchecks.

Describe the solution you'd like Use sd_notify mechanism to communicate application status to systemd.

Describe alternatives you've considered Using a wrapper process to poll for status and notify systemd, like so: https://www.medo64.com/2019/01/systemd-watchdog-for-any-service/

[sbienkow-ninja]

Needs GH-330 to be done first.