Public API: Allow extra-metadata in DataContext

interface ExtendedDataContext extends DataContext {
    MetaDataContext meta();
    DataContext data();
}
// then:
MetaDataContext meta = MapDataContext.create();
ctx.set("some-flag", value);
MyCustomCtx datactx = new MyCustomCtx(...,...,...); 
DataContext ctx = ExtendedDataContext.create(meta, datactx);
// the service decodes the type of context
DataContext res = dmnSvc.evaluate(someId, ctx); 
ExtendedDataContext ext = res.as(ExtendedDataContext.class);
MetaDataContext = ext.meta() // may be empty
MyCustomCtx tres = ext.data().as(MyCustomCtx.class) // extract a typed result