Implement User Commands for Satellite

This MR adds the possibility to register user commands from the satellite interface and is based on !140 (merged) which needs merging first.

User commands

• can have any number of arguments which have to be capable of conversion to their type from config::Value (i.e. anything we could also send in a configuration dictionary). The arguments need to be provided in a List, i.e. a std::vector<config::Value.
• have a description that is returned with get_commands and describes the call
• can be limited to run only in specific FSM states

Especially the latter is a cool feature I think since the user can avoid having config-changing commands be executed e.g. during run.

Documentation is provided in the MR in the form of a how-to guide.