add filterable values

Author: Alejo

plugins/woocommerce/src/main/kotlin/org/wordpress/android/fluxc/network/rest/wpcom/wc/product/CoreProductStockStatus.kt

@@ -13,6 +13,7 @@ enum class CoreProductStockStatus(val value: String) {
         private val valueMap = entries.associateBy(CoreProductStockStatus::value)
         val ALL_VALUES = valueMap.keys
 
+        val FILTERABLE_VALUES = setOf(IN_STOCK, OUT_OF_STOCK, ON_BACK_ORDER)
         /**
          * Convert the base value into the associated CoreProductStockStatus object
          */