Implement a "side door" mechanism in the daemon to remotely configure a MuonPi behind firewalls

[hangeza]

Description
The idea of simplifying the configuration of a remote MuonPi detector with a so-called "side door" has long existed. This requires sending configuration data towards the detector, even if it is located in a private subnet behind a firewall. This should certainly be set up in such a way that this only happens with the consent of the detector user (e.g. checkbox in the GUI) and the general safety is guaranteed. In the current layout of the MuonPi project the already existing MQTT link would be suitable, on which the detector appears as a publisher under the topics "muonpi / data" and "muonpi / log", i.e. only the downstream from the detector to the server is implemented.

Proposal
Implement an upstream channel (towards the detector) via the MQTT link such that configuration data can be published into the MQTT system targetting a specific detector (by site_id). The daemon will have to subscribe to a corresponding topic, say muonpi/config/<site_id> for this to work.

Notes:

• The config channel shall be deactivated by default. It can be activated locally by the detector user through the GUI or a CLI command.
• A program which acts as a publisher for the config topic has to be created. This may be a dedicated GUI or, in ideal case, implemented into the existing GUI program.