[generator] Add support for nullable annotations in api definitions

[rolfbjarne]

Currently the generator doesn't support nullable annotations (?) in api definitions, if something is nullable, it has to be annotated with the [NullAllowed] attribute.

We should add support to (and potentially switch our code to use) nullable annotations instead.

[GalaxiaGuy]

Would that include support for the many NRT related attributes, so issues like this can be fixed?
#13416

[rolfbjarne]

Yes, we could look at that too.

[rolfbjarne]

This might be mostly fixed with 0679876.

[rolfbjarne]

One thing that's missing is propagating nullability information for generic types.

macios/src/bgen/AttributeManager.cs

Lines 659 to 662 in 70046dc

	// Getting complex nullability right means we'll have to completely rework how we render types.
	// So don't do that for now, just look at the outermost type (the first number in the array),
	// and return nullability depending on that value.
	return ((byte) valueCollection [0].Value) == 2;

This typically shows up in delegates, like here for instance:

macios/src/gamekit.cs

Line 1124 in 70046dc

Action<UIViewController, NSError> AuthenticateHandler { get; set; }

which should be:

public virtual Action<UIViewController?, NSError?> AuthenticateHandler

which doesn't work right now.