[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.