FieldLengthAttribute where length value includes size of field that stores length

[lOlbas]

Consider the following payload:

          00 01 02 03 04 05
00000000  06 00 01 02 03 04

First two bytes indicate the size of the entire payload, not just the data after it. This is the way I would describe this message:

public class RootEvent
{
    [FieldOrder(1)]
    public ushort DataLength;

    [FieldOrder(2)]
    [FieldLength(nameof(DataLength), -sizeof(ushort)] // Second parameter is "adjustment" for the actual length
    public object Data;
}

However I don't seem to find a way to implement something similar using this library, did I miss something? 🤔