Support marshaling System.Guid from C#

[warriordog]

The System.Guid type cannot be marshaled from C# to Godot. This makes it difficult to store UUIDs in object metadata, resources, or anything else managed by the engine. To marshal a Guid, it must be converted to/from some other type like string. This works, but can become very clunky when Guids must be passed back-and-forth between C# and Godot. There's also a significant parsing overhead versus just passing raw bytes around.

If Guid could be marshaled directly, then those issues disappear. I can think of a few different ways to implement this:

1. Marshal to TYPE_RAW_ARRAY - This is the easiest and quickest way to implement this. Just marshal the Guid (which is just a 16 byte struct) to a raw byte array and leave everything else up to scripts.
2. Marshal to a new TYPE_UUID - This is more complex, but would be the cleanest solution. First implement a proper UUID type in Godot, then allow marshaling it to System.Guid. Doing this "right" would mean also exposing the new UUID type and associated logic to GDScript.
3. Marshal to a new TYPE_INT_128 - Yet another approach is to support 128-bit integers and marshal to that, but this would require complete software emulation of the type. I personally don't think there's enough uses for that type to be worth it, but I'm putting it here for completeness.