[12.x] Allow Stripe dashboard subscriptions

[driesvints]

Allows subscriptions to be created from the Stripe dashboard. The newSubscriptionName allows users to configure a different default key when overriding the WebhookController.

The only limitation with this PR is that it won't work in situations where you have more than one type of subscription and need to differentiate between the names.

[LukeTowers]

@driesvints is there anything preventing the newSubscriptionName() method from evaluating the payload to identify the products / prices in the subscription provided by stripe and using an internal mapping of that to dynamically return a subscription name for Cashier to use?

[driesvints]

@LukeTowers heya. Not really. Once you have the payload you can do whatever you want with it I think.

[LukeTowers]

@driesvints awesome, thanks! I thought that would be the case, was just asking because of your extra note in the original PR:

The only limitation with this PR is that it won't work in situations where you have more than one type of subscription and need to differentiate between the names.

[driesvints]

@LukeTowers ah yeah sorry. In case you have multiple subscription types, for example, a tennis and a gym subscription, you can't derive the name from the subscription alone because there's no way to set metadata when subscribing a user from the dashboard. But you could indeed derive it from the prices like you said, that's a good shout.