Allow default value encoded as JSON or as the default constructor

Consider this :
```
class InnerClass {
int field = 0;
int otherField = 0;
}

class OuterClass {
InnerClass innerClass = InnerClass();
// other fields
}
```

I can't set a default value for `innerClass`. I can edit the generated file of course, but that's annoying.
Dart semantics don't allow non-const values in annotations from my understanding, so a specific annotation field such as `includeFromJson` would be required.