Add braintree_nonce to support spree_braintree_vzero.

[mdavo6]

Is your feature request related to a problem? Please describe.

I would like to add support for Braintree payments to Vue Spree (vuestorefront-community/spree#261), using the spree_braintree_vzero gem. Vue Spree currently utilises v4.11.0 of spree-storefront-api-v2-js-sdk. The spree_braintree_vzero gem requires that a nonce be sent from the frontend to spree, however the Payment.ts interface only supports payment_method_id as an attribute.

Describe the solution you'd like.
Add a new optional string attribute called braintree_nonce to IPayment.ts.