cassanknex doesn't exist on FeathersCassandra

Relatively simple error, `cassanknex` is defined on the imported `feathers-cassandra` module. Just looking at the index.d.ts file, cassanknex just needs to be added to the `CassandraServiceOptions` interface.
```
Property 'cassanknex' does not exist on type '<T>(config?: Partial<CassandraServiceOptions> | undefined) => Service<T>'.
```