feat: add support for braintree payments

[mdavo6]

Add support for Braintree payments drop in UI - requires spree_braintree_vzero gem.

Notes:

1. API-client is currently failing as braintree_nonce is not currently a supported type in spree-storefront-api-v2-js-sdk. I have submitted a PR to address this: Add braintree nonce to IPayment to support spree_braintree_vzero spree/spree-api-v2-js-sdk#346
2. Requires the new Braintree token generation API endpoint provided in the following PR: Add new API v2 endpoint to support Braintree Token generation spree-contrib/spree_braintree_vzero#272

Description

• Added new getBraintreeToken API call.
• Added braintree nonce to payment attributes (as required by spree_braintree_vzero gem).
• Added braintreetoken type.
• Added new braintree.vue payment method component.
• Added braintree-web-drop-in as new dependency (required by braintree.vue payment method component).

Related Issue

#260

Motivation and Context

Braintree is a popular payment gateway in the Spree community. Supporting Braintree out of the box will offer merchants greater options, including offering Paypal as a payment method.

How Has This Been Tested?

Tested on chrome.

Screenshots (if appropriate):

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation. (Will need to update readme document if approved).
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[rafalcymerys]

@mdavo6 thanks, this looks solid! @letelete would you like to take a look as well?