Expand navigations on grouping key

[smitpatel]

Enabling group by entity type allows navigations to be written on the grouping key.
e.g.

context.Customers.GroupBy(c => c.Navigation).Select(g => g.Key.Navigation2)

These navigations on grouping key cannot be expanded/translated in normal way because normal path will try to add Join after GroupBy, which doesn't translate to SQL. If we add join before GroupBy then it still won't be available in selector after grouping. All navigations on grouping key required to be created using subquery form using correlation predicate (and SingleOrDefault if reference navigation)