LOOKUP (dimension table) does not work with GROUP BY

From the broker logs:

```log
2023/06/30 21:28:35.954 ERROR [PinotClientRequest] [jersey-server-managed-async-executor-1] Caught exception while processing POST request
java.lang.IllegalArgumentException: Unsupported function: lookup not found
at org.apache.pinot.core.query.postaggregation.PostAggregationFunction.(PostAggregationFunction.java:47)
```

It's called out in the docs that `LOOKUP` is not supported for multi-value fields. But is it intentionally not supported for post-aggregation decoration? Or is this just an omission?

Start with org.apache.pinot.core.query.postaggregation.PostAggregationFunction.java, named in the broker stack trace, and trace how LOOKUP is resolved for post-aggregation expressions in GROUP BY queries. Compare that path with the documented multi-value limitation and determine whether the expected result is support or an explicit restriction; done means the behavior is defined and verified without the current unsupported-function error.

Written by the indexing model from the issue text.