Handle dispatcher serialization is currently inflexible and cannot be extended without breaking backwards-compatibility.

We should introduce a new dispatcher header type that is more flexible and begin using it only for dispatchers which need to serialize new features (which old EDK versions will of course be unable to use anyway).

The legacy dispatcher header format will still be used for all other dispatchers until such a time as ARC++ can uprev libmojo to a version which supports the new format. At that point we can fully remove support for legacy dispatchers.