implement more of MAX-3B-compatible REST API

[jeffkowalski]

Consider implementing more REST API points described in
https://github.com/ki6syd/minimalist-amateur-xcvr/tree/main/api, and https://app.swaggerhub.com/apis-docs/KI6SYD_1/MAX-3B/1.0.2

Per Brian, we may choose to create different entry points since the document is currently MAX-3B specific (for example, it has RxBandwidth setting but not an independent Mode setting, and for a KX radio we want Mode and would rarely want to change RxBandwidth).