DoFn ProcessContext needs a MultiSideInput interface

We currently have the SideInput annotation, but we also need this interface for multiple side inputs.
```

public interface MultiSideInput {
/** Returns the value of the side input. * */
T sideInput(PCollectionView
view);
}

```

Imported from Jira [BEAM-14085](https://issues.apache.org/jira/browse/BEAM-14085). Original Jira may contain additional context.
Reported by: apilloud.