Allow to optionally create token with customer_id option

[skukx]

Right now Solidus::Gateway::BraintreeGateway model allows options to be passed to the generate_client_token method when generating the braintree token.

solidus_braintree/app/models/solidus/gateway/braintree_gateway.rb

Line 47 in d9c8f26

def generate_client_token(options = {})

However when calling the api to call generate the token. No options are passed or considered
https://github.com/solidusio/solidus_braintree/blob/d9c8f26f941d912c482ba3b98934f331c50370ac/app/controllers/spree/api/braintree_client_token_controller.rb

This means every card is created on a new Braintree Customer. If a user is logged in, that user should belong to 1 Braintree Customer and all cards created should be attached to that same customer. The hosted fields right now does not support this as far as I can see.

This feature becomes extremely important with braintree recurring billing and updating a payment method. The update will fail because the newly created card belongs to another customer and not the same customer as on the subscription.

The token creation would look something like:

Braintree::ClientToken.generate customer_id: <customer id>