Support user id filtering in subscriptions list

[strixcuriosus]

Is your feature request related to a problem? Please describe.
The SubsciptionsClient.list_subscriptions appears to yield all visible subscriptions within an org. The Subscriptions API has support for a user_id parameter to filter, but this does not yet appear to have support in the client.

Describe the solution you'd like

• add support for optional user_id filter arg to SubsciptionsClient.list_subscriptions
• add a utility to make it easy to lookup or get your own user_id

Describe alternatives you've considered
instead of separating get user id and user id filtering, there could be an arg such as owned_by_current_user. this would be easier to use but less directly mapped to the API params

Additional context
looks like someone left a TODO to add user_ud support here already https://github.com/planetlabs/planet-client-python/blob/main/planet/clients/subscriptions.py#L78