[#1741] Change MutinyGenerator#generate return type

We `Uni<?>` we can avoid an additional redundant conversion
before returning the value in some situations.

For example, if the return value is the result of a native
query:
```
return session.createNativeQuery("...", Long.class).getSingleResult();
```

This is useful because with native queries we don't always know in advance
what value will be returned. And using Object.class might cause
ClassCastExceptions along the way.

Author: DavideD

hibernate-reactive-core/src/main/java/org/hibernate/reactive/tuple/MutinyGenerator.java

@@ -21,6 +21,5 @@ public Object generate(SharedSessionContractImplementor session, Object owner, O
         throw new UnsupportedOperationException( "Use generate(Mutiny.Session, Object, Object, EventType) instead" );
     }
 
-    public abstract Uni<Object> generate(Mutiny.Session session, Object owner, Object currentValue,
-                                         EventType eventType);
+    public abstract Uni<?> generate(Mutiny.Session session, Object owner, Object currentValue, EventType eventType);
 }