ot::Utils::CmdLineParser::ParseAsHexString() function

Parses 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. Correctly handles hex strings with even or odd length. For example, "AABBCCDD" (with even length) is parsed as {0xaa, 0xbb, 0xcc, 0xdd} and "123" (with odd length) is parsed as {0x01, 0x23}.