feat(checkout): dynamic checkout fields with PHP filter support

Backend (CheckoutController):
- Enhanced get_fields() API with custom_attributes, search_endpoint,
  search_param, min_chars, input_class, default
- Supports new 'searchable_select' field type for API-backed search

Customer SPA:
- Created DynamicCheckoutField component for all field types
- Checkout fetches fields from /checkout/fields API
- Renders custom fields from PHP filters (billing + shipping)
- searchable_select type with live API search
- Custom field data included in checkout submission

This enables:
- Checkout Field Editor Pro compatibility
- Rajaongkir destination_id via simple code snippet
- Any plugin using woocommerce_checkout_fields filter

Updated RAJAONGKIR_INTEGRATION.md with code snippet approach.

includes/Api/CheckoutController.php

@@ -475,6 +475,14 @@ class CheckoutController {
                     'custom'      => !in_array($key, $this->get_standard_field_keys()), // Flag custom fields
                     'autocomplete'=> $field['autocomplete'] ?? '',
                     'validate'    => $field['validate'] ?? [],
+                    // New fields for dynamic rendering
+                    'input_class'       => $field['input_class'] ?? [],
+                    'custom_attributes' => $field['custom_attributes'] ?? [],
+                    'default'           => $field['default'] ?? '',
+                    // For searchable_select type
+                    'search_endpoint'   => $field['search_endpoint'] ?? null,
+                    'search_param'      => $field['search_param'] ?? 'search',
+                    'min_chars'         => $field['min_chars'] ?? 2,
                 ];
             }
         }