sched_ext: Move internal type and accessor definitions to ext_internal.h

sched_ext: Move internal type and accessor definitions to ext_internal.h

There currently isn't a place to place SCX-internal types and accessors to
be shared between ext.c and ext_idle.c. Create kernel/sched/ext_internal.h
and move internal type and accessor definitions there. This trims ext.c a
bit and makes future additions easier. Pure code reorganization. No
functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Andrea Righi <arighi@nvidia.com>