Fix multiple N+1 on /admin/customers.json

This (should) considerably improve traces like
https://app.datadoghq.com/apm/trace/917632173599137280?spanID=3163385094622710144&env=production&sort=time&colorBy=service&spanViewType=metadata&graphType=flamegraph&shouldShowLegend=true
by fixing the following 3 N+1s

```
user: root
GET /admin/customers.json?enterprise_id=4
USE eager loading detected
  Customer => [:enterprise]
  Add to your query: .includes([:enterprise])
Call stack
  /usr/src/app/app/serializers/api/admin/customer_with_calculated_balance_serializer.rb:24:in `balance_value'
  /usr/src/app/app/serializers/api/admin/customer_with_calculated_balance_serializer.rb:9:in `balance'
  /usr/src/app/app/controllers/admin/customers_controller.rb:20:in `block (2 levels) in index'
  /usr/src/app/app/controllers/admin/customers_controller.rb:17:in `index'

user: root
GET /admin/customers.json?enterprise_id=4
USE eager loading detected
  Spree::Address => [:state]
  Add to your query: .includes([:state])
Call stack
  /usr/src/app/app/serializers/api/address_serializer.rb:14:in `state_name'
  /usr/src/app/app/controllers/admin/customers_controller.rb:20:in `block (2 levels) in index'
  /usr/src/app/app/controllers/admin/customers_controller.rb:17:in `index'

user: root
GET /admin/customers.json?enterprise_id=4
USE eager loading detected
  Spree::Address => [:country]
  Add to your query: .includes([:country])
Call stack
  /usr/src/app/app/serializers/api/address_serializer.rb:10:in `country_name'
  /usr/src/app/app/controllers/admin/customers_controller.rb:20:in `block (2 levels) in index'
  /usr/src/app/app/controllers/admin/customers_controller.rb:17:in `index'
```

This popped up after improving the balances calculation. Now, that it's
fast, it's clear that are more performance problems on that endpoint.
We'll see if there are any left after this.

app/controllers/admin/customers_controller.rb

@@ -74,7 +74,12 @@ module Admin
     def collection
       if json_request? && params[:enterprise_id].present?
         customers_relation.
-          includes(:bill_address, :ship_address, user: :credit_cards)
+          includes(
+            :enterprise,
+            { bill_address: [:state, :country] },
+            { ship_address: [:state, :country] },
+            user: :credit_cards
+          )
       else
         Customer.where('1=0')
       end