Support for nested keyfields for Apollo-Client-Helpers

**Is your feature request related to a problem? Please describe.**
Looking at book typepolicy keyfields example from the apollo client docs https://www.apollographql.com/docs/react/caching/cache-configuration/#generating-unique-identifiers you can see there is support for nested fields. However the types that get generated from the apollo-client helpers don't seem to support this yet and gives an error where you try to use a nested keyfield.

**Describe the solution you'd like**

support for nested keyfields.

Apollo-Client Helpers looks like a great addition to the already amazing code generator 👍