Add exported IsNullable method to nullable types

[Kalbi]

I was very happy to see #430 merged and am now using it. However, in my project, I felt that having an exported IsNullable method would be useful, especially when implementing Nullable custom scalar types.

This will allow one to verify that custom Nullable implementations correctly implement the NullUnmarshaller interface.

It also allows us to verify the type of the input field for things like gRPC field masks.

E.g., previously, if one had a traditional nullable field, we would not be able to generate gRPC field masks accurately for partial updates because if a field was nil we would not add the field mask. Now, users can check if a field IsNullable, and if it is, check if the Set field is true or false, in order to set a field mask accurately.

Hopefully this method is safe to export; it would be helpful for our project and I hope it will be useful to others who want to take advantage of the new Nullable functionality.

[pavelnikolov]

Hi @Kalbi,
all these types already have a public Nullable() {} method. This means that you can create an interface that has this method and test for yourself if the type implements it. I am not convinced that we need to expose an additional static method IsNullable. This doesn't add any functionality that is not already available. At the cost of one additional interface, you can check for yourself.