Can't use ENTITY_REFERENCE event for resolution in an Attribute

I need to resolve custom XML entities in some custom event handler/function. Sadly, an unknown entity doesn't trigger the `XMLEvent.ENTITY_REFERENCE` event. If this feature is not implemented, is there any workaround?

Here is a test case showing what I expect from Aalto. I'm using aalto-xml 1.1.0.

```
package com.otcdlink.chiron.wire;

import org.junit.jupiter.api.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamReader;
import javax.xml.stream.events.XMLEvent;
import java.io.StringReader;

import static org.junit.jupiter.api.Assertions.fail;

public class StaxPlayground {

@Test
void entityReplacement() throws XMLStreamException {
final XMLInputFactory xmlInputFactory =
new com.fasterxml.aalto.stax.InputFactoryImpl() ;
// javax.xml.stream.XMLInputFactory.newInstance() ;

xmlInputFactory.setProperty( XMLInputFactory.SUPPORT_DTD, true ) ;
xmlInputFactory.setProperty( XMLInputFactory.IS_REPLACING_ENTITY_REFERENCES, false ) ;
xmlInputFactory.setProperty( XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES, false ) ;
xmlInputFactory.setProperty( XMLInputFactory.IS_COALESCING, false ) ;

final String xml =
"\n" +
""
// ""
;

final XMLStreamReader xmlStreamReader =
xmlInputFactory.createXMLStreamReader( new StringReader( xml ) ) ;

found: {
while( xmlStreamReader.hasNext() ) {
final int staxEvent = xmlStreamReader.next() ;
if( staxEvent == XMLEvent.ENTITY_REFERENCE ) {
LOGGER.info( "Got entity reference '" + xmlStreamReader.getLocalName() + "'." ) ;
break found ;
}
}
fail( "Found no entity reference." ) ;
}
}

// =======
// Fixture
// =======

private static final Logger LOGGER = LoggerFactory.getLogger( StaxPlayground.class ) ;
}
```

All I get is an exception:

```
com.fasterxml.aalto.WFCException: Unexpanded ENTITY_REFERENCE (replace-me) in attribute value
at [row,col {unknown-source}]: [2,26]

at com.fasterxml.aalto.in.XmlScanner.reportInputProblem(XmlScanner.java:1333)
at com.fasterxml.aalto.in.XmlScanner.reportUnexpandedEntityInAttr(XmlScanner.java:1343)
at com.fasterxml.aalto.in.ReaderScanner.collectValue(ReaderScanner.java:901)
at com.fasterxml.aalto.in.ReaderScanner.handleStartElement(ReaderScanner.java:794)
at com.fasterxml.aalto.in.ReaderScanner.nextFromProlog(ReaderScanner.java:236)
at com.fasterxml.aalto.stax.StreamReaderImpl.next(StreamReaderImpl.java:790)
at com.otcdlink.chiron.wire.StaxPlayground.entityReplacement(StaxPlayground.java:44)
```