ot::Utils::CmdLineParser::Arg::ParseAsHexString() const method

Parses the argument as a hex string into a byte array of fixed expected size. Returns `kErrorNone` only when the hex string contains exactly @p aSize bytes (after parsing). If there are fewer or more bytes in hex string that @p aSize, the parsed bytes (up to @p aSize) are copied into the `aBuffer` and `kErrorInvalidArgs` is returned.