sql-api SqlToStream type issue

`SqlToStreamOperator` extends `UnaryToUnaryOperator` which requires specifying a generic input and output. For `SqlToStreamOperator` this is set by default to `SqlToStreamOperator` however this causes a crash for the `JoinOperator` in the sql-api as we flatten the `tuple2` result after joining. In practice this can lead to cases where if we have a pipeline like:
`postgres join -> SqlToStream -> java flatten map`
This will crash due to `SqlToStream` forcing the `tuple2` to be a `Record`.