Support composite partition key as part of composite primary key

Enhance the ColumnFamilyCreate generator to support a composite partition key by making the partition key an array of columns, e.g.

```ruby
create_table :foo, {
  primary_key: [[:id, :version], :field],
  columns: {
    id: :text,
    version: :text,
    field: :text
  }
}
```

Will generate the CQL `create table foo (id text, version text, field text) PRIMARY KEY ((id, version), field);`