Add public accessor to for `is_single_entity` to Query class

Add public accessor to for `is_single_entity` to Query class

Fixes: #4934
This is useful when writing middleware between your application
and sqlalchemy, to inspect what type of results we will be seeing.

As an example,

```
def issue_query_and_wrap_results(query, Wrapper):
  results = query.all()
  for result in results:
    if q.is_single_entity:
      return Wrapper(result)
    else:
      return tuple(Wrapper(r) for r in result)
```

Without this accessor, you need to inspect the return results or private members of the
query and try to guess the intent