Create an example in the examples directory that adds a custom plugin that adds a requiredTypename() method on interface and union refs.

The idea is to just return the same ref, but update the types so that __typename: string is a required property for any fields returning that union or interface
The example should have the full plugin with types and the method implementation attached to class proptotypes on a single plugin file and an example schema that can be used to ensure it type checks and works with a simple test.