Publish a read-write field as read-only in the web service

[Leonard Richardson]

We have a way to publish a read-only attribute as read-write (by defining a mutator method that's invoked when the client tries to modify the attribute). We don't have the opposite: a way to prohibit writes through the webservice on a read-write field.

The original use case for this was IBug.private. The IBug.private problem was fixed another way, but it would have been easier to fix if we had @expose_as_readonly. The next time we have this kind of use case, let's fix it this way.