Multiple batch joins on the same key results in multiple shuffles

The query like this:

```

SELECT a.*, b.*, c.* FROM a JOIN b ON a.some_id = b.some_id JOIN c ON a.some_id = c.some_id;

```

results in two shuffles. Can probably be optimized.

Relevant code:

- BeamJoinRel implements Join in SQL: https://github.com/apache/beam/blob/1675b0f843ed34de8ba6f3676f794db80b40139d/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/impl/rel/BeamJoinRel.java#L194

- CoGBK Join implementation: https://github.com/apache/beam/blob/279a05604b83a54e8e5a79e13d8761f94841f326/sdks/java/extensions/join-library/src/main/java/org/apache/beam/sdk/extensions/joinlibrary/Join.java#L36

Imported from Jira [BEAM-5049](https://issues.apache.org/jira/browse/BEAM-5049). Original Jira may contain additional context.
Reported by: kedin.