Can't use side inputs with Combine.PerKeyWithHotKeyFanout

PerKeyWithHotKeyFanout doesn't allow a user to specify a side input and access it in the CombineFn. When the function `withHotKeyFanout` is called, any sideinputs stored in the Combine.PerKey are dropped. ([https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Combine.java#L1571](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Combine.java#L1571)): 
```

public PerKeyWithHotKeyFanout withHotKeyFanout(
SerializableFunction hotKeyFanout) {
return new PerKeyWithHotKeyFanout<>(fn, fnDisplayData, hotKeyFanout);

}

```

We would like to be able to use side inputs and hot key fanout.

Imported from Jira [BEAM-11005](https://issues.apache.org/jira/browse/BEAM-11005). Original Jira may contain additional context.
Reported by: miagilepner.