Implement Hamlib rigctld network protocol

[brianmathews]

Currently the SOTACAT implements its own REST API for clients to issue commands to it, such as "set VFO frequency" or "get VFO frequency". The SOTACAT translates these REST API calls into radio specific (KX2/KX3) CAT commands such as "FA;".

The HTML/Javascript application that is served by the SOTACAT to the phone's browser knows how to talk to that REST API. However, no other programs on the planet (besides the SOTACAT HTML and SOTAMAT) know how to speak that proprietary REST API.

This is a suggestion to change the REST API to instead use the "standard" Hamlib rigctld network protocol. There are many programs that already know how to control various radios using the rigctld protocol over TCP/IP, and how to issue commands such as "set VFO frequency", "get VFO frequency" etc.

Implementing a standard protocol could allow many applications to control the KX2/KX3 wirelessly.

We will need to extend the rigctld protocol to include concepts that are specific to the SOTACAT such as "direct FT8 synthesis" for SOTAMAT, and only the SOTACAT UI and SOTAMAT will understand those extensions.