Add a Planetary Variable source helper to the subscription_request module

[sgillies]

To generate source objects like the one shown at https://developers.planet.com/docs/subscriptions/pvs-subs/#create-a-planetary-variables-subscription. It should be based on the existing catalog_source() helper https://github.com/planetlabs/planet-client-python/blob/main/planet/subscription_request.py#L99.