add auto_recurse option to selectinload, immediateload

add auto_recurse option to selectinload, immediateload

Added very experimental feature to the :func:`_orm.selectinload` and
:func:`_orm.immediateload` loader options called
:paramref:`_orm.selectinload.auto_recurse` /
:paramref:`_orm.immediateload.auto_recurse` , which when set to True will
cause a self-referential relationship load to continue loading with
arbitrary depth until no further objects are found. This may be useful for
self-referential structures that must be loaded fully eagerly, such as when
using asyncio.

Fixes: #8126
Change-Id: I5bbd00bd0ca43f4649b44680fea1e84680f0a5db