Support systemd status and control

[SamuelDudley]

Create a graphql interface to maverick systemd. This will allow the user to check the current status of maverick services from maverick-web.

suggest polling systemd via https://github.com/emlid/systemd-manager and periodically (0.5Hz) send updates to the client via graphql.

The list of supported services could be populated from the contents of glob /etc/systemd/system/maverick* and potentially custom paths that are controlled via a config file

[SamuelDudley]

Currently this is rejected in place of process calls