Adds support for using primitive types in joins.

BREAKING CHANGE: Some existing joins may now fail and the type of `ROWKEY` in the result schema of joins may have changed.

When `ROWKEY` was always a `STRING` it was possible to join an `INTEGER` column with a `BIGINT` column.  This is no longer the case. A `JOIN` requires the join columns to be of the same type. (See confluentinc#4130 which tracks adding support for being able to `CAST` join criteria).

Where joining on two `INT` columns would previously have resulted in a schema containing `ROWKEY STRING KEY`, it would not result in `ROWKEY INT KEY`.