ot::Utils::CmdLineParser::ParseCmd() function

Parses a given command line string and breaks it into an argument list. May change the input @p aCommandString, it will put a '\0' by the end of each argument, and @p aArgs will point to the arguments in the input @p aCommandString. Backslash ('\') can be used to escape separators (' ', '\t', '\r', '\n') and the backslash itself. As the arguments are parsed, the @p aArgs array entries are populated. Any remaining @p aArgs entries in the array will be cleared and marked as empty. So the number of arguments can be determined by going through @p aArgs array entries till we get to an empty `Arg` (i.e., `Arg::IsEmpty()` returns `true). Ensures that the last entry in @p aArgs array is always used to indicate the end (always marked as empty), so the @p aArgs array should have one more entry than the desired max number of arguments.