add form_fields to Customer

- vendor open PR from bigcommerce#156
- replacing existing vendor of https://github.com/watermelonexpress/bigcommerce-api-ruby (renaming repo) to ensure the fork remote is bigcommerce's rather than a github user's.

Form fields exist on Customer but are missing from the customer resource object.

Schema definition:

  $ curl https://developer.bigcommerce.com/api-reference/store-management/customers-v2/BigCommerce_Customers_API.oas2.json | jq '.definitions.Customer.properties.form_fields'

  {
    "description": "Array of custom fields. This is a READ-ONLY field; do not set or modify its value in a POST or PUT request.",
    "type": "array",
    "items": {
      "title": "Form Fields",
      "type": "object",
      "properties": {
        "name": {
          "description": "Name of the form field",
          "type": "string",
          "example": "License Id"
        },
        "value": {
          "description": "Value of the form field",
          "type": "string",
          "example": "123BAF"
        }
      }
    }
  }

lib/bigcommerce/resources/customers/customer.rb

@@ -28,6 +28,7 @@ class Customer < Resource
     property :addresses
     property :tax_exempt_category
     property :accepts_marketing
+    property :form_fields
 
     def self.count(params = {})
       get 'customers/count', params