Command struct representing a command and its variable types.

Represents both a group and a single command. A group is characterized by the its argument list being { SL_CLI_ARG_GROUP }. The handler will then point to the first element in the group array (sl_cli_command_entry_t[]). To initialize, use macros SL_CLI_COMMAND or SL_CLI_COMMAND_GROUP. arg_help is string where 0x1f (unit separator) separates the messages.