add session-wide execution_options

add session-wide execution_options

Added support for per-session execution options that are merged into all
queries executed within that session. The :class:`_orm.Session`,
:class:`_orm.sessionmaker`, :class:`_orm.scoped_session`,
:class:`_ext.asyncio.AsyncSession`, and
:class:`_ext.asyncio.async_sessionmaker` constructors now accept an
:paramref:`_orm.Session.execution_options` parameter that will be applied
to all explicit query executions (e.g. using :meth:`_orm.Session.execute`,
:meth:`_orm.Session.get`, :meth:`_orm.Session.scalars`) for that session
instance.

Fixes: #12659
Change-Id: I6e19e1567e0c04df32ba1d43baf420fb762f155c