Allow creating a subscription via the API

[aldesantis]

Once all the information a subscription needs can be set on the subscription without having to be copied from the order, we can give users the ability to create a subscription directly via the API, outside of the regular order creation flow.

This simplifies the integration very much, especially for stores that only offer subscriptions.

Blocked by #116 and #117.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It might be closed if no further activity occurs. Thank you for your contributions.