Add create subscription API

[allentown521]

There is a common scenario in saas products, which is free trial. In stripe, we usually create a subscription for the customer (unpaid subscription) by calling the create subscription api. The advantage is that the user does not need to add payment information to start the trial. After the trial expires, the customer can pay in the portal.

If there is no such API, the entire process needs to be implemented by itself. We have to manually manage the trial period, otherwise the user can only pay first and then use it

[allentown521]

any plan?