C++: Check Output File, send Disk Space Telemetry

This adds a method to the ReceiverSatellite which allows checking a configured output file for validity (path exists, can be written to, file does not exist already). It also registers the DISKSPACE_FREE metric which transmits the available disk space at the target path in MB. It also logs WARNING or CRITICAL messages when disk space gets low, to also alarm via CMDP LOG.

To do:

• Rename method
• Maybe add other method which opens and returns an fstream?
• Adjust parameters & checks to requirements in Constellation
• Think about how to incentivize developers to use this.

This implements part of #84 (closed)

Documentation includes monitoring examples: