populate existing can be in exec option in session.get

The ``populate_existing`` execution option is now honored when passed
in the :paramref:`.Session.get.execution_options` dict by the method
:meth:`.Session.get` and analogous in other session kinds. The current
:paramref:`.Session.get.populate_existing` parameter will takes precedence
if specified, overriding the value of the execution options.

Fixes: #10610
Change-Id: I4ddc9a7c6dda8f31f4dd413b49a9196efb3edaa6