Add MSP2_COMMON_SETTING for retrieving arbitrary settings and
MSP2_COMMON_SET_SETTING for setting them. This exposes any setting
which was only available from the CLI via MSP now.
Add support in settings.rb to generate a JSON file with all the
settings, their types and their possible values for settings using
a table. This file can be used by clients to properly format
messages for settings over MSP.