BraintreePlugin: Allow customer to specify whether to save card

[michaelbromley]

Is your feature request related to a problem? Please describe.
When using the BraintreePlugin with the storeCustomersInBraintree option set to true, the payment handler will automatically persist any card data to the Braintree vault upon successful CC payment.

We should support an option that allows the customer to opt-out of this behaviour.

Describe the solution you'd like
In the metadata argument of the addPaymentToOrder handler, we can add support for a storeCardInVault: boolean; property which can be used to control this. For reasons of backward-compatibility it should default to true where we have a braintree customerId already stored for that customer.

[michaelbromley]

Turns out that is this not possible to control in the backend. See this discussion:

• storeInVault option seems to have no effect braintree/braintree_node#208

So the solution needs to be implemented on the client side (storefront) by not passing the customerId when creating the dropin instance.