Enable Session to contain both primitive.run and backend.run

[merav-aharoni]

What is the expected feature or enhancement?
After the implementation of backend.run is complete, we would like to be able
to run both primitive.run and backend.run in a single session. See #1138.

Acceptance criteria
Be able to run something like:

backend = service.backend('ibmq_qasm_simulator')

with Session(service, backend=backend) as session:
    sampler = Sampler(session=session)
    
    job1 = sampler.run(bell)
    job2 = backend.run(bell) # this would not run in a session