Added loader options to session.merge, asyncsession.merge

Added loader options to session.merge, asyncsession.merge

Added loader options to :meth:`_orm.Session.merge` and
:meth:`_asyncio.AsyncSession.merge`, which will apply the given loader
options to the ``get()`` used internally by merge, allowing eager loading
of relationships etc. to be applied when the merge process loads a new
object. Pull request courtesy Daniel Stone.

Fixes: #6955
Closes: #6957
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/6957
Pull-request-sha: ab4d96cd5da9a5dd01112b8dcd6514db64aa8d9f

Change-Id: I5b94dfda1088a8bc6396e9fd9a072827df1f8680