add graphql-calculator (#118)

Author: dugenkui03

src/content/code/language-support/java-kotlin-android/server/graphql-calculator.md

@@ -0,0 +1,57 @@
+---
+name: graphql-calculator
+description: A lightweight graphql calculation engine.
+url: https://github.com/graphql-calculator/graphql-calculator
+github: graphql-calculator/graphql-calculator
+---
+
+GraphQL Calculator is a lightweight graphql calculation engine,
+which is used to alter execution behavior of graphql query.
+
+Here are some examples on how to use GraphQL Calculator on graphql query.
+
+```graphql
+query basicMapValue($userIds:[Int]){
+    userInfoList(userIds:$userIds)
+    {
+        id
+        age
+        firstName
+        lastName
+        fullName: stringHolder @map(mapper: "firstName + lastName")
+    }
+}
+query filterUserByAge($userId:[Int]){
+    userInfoList(userIds: $userId)
+    @filter(predicate: "age>=18")
+    {
+        userId
+        age
+        firstName
+        lastName
+    }
+}
+query parseFetchedValueToAnotherFieldArgumentMap($itemIds:[Int]){
+        itemList(itemIds: $itemIds){
+            # save sellerId as List<Long> with unique name "sellerIdList"
+            sellerId @fetchSource(name: "sellerIdList")
+            name
+            saleAmount
+            salePrice
+        }
+        userInfoList(userIds: 1)
+        # transform the argument of "userInfoList" named "userIds" according to expression "sellerIdList" and expression argument, 
+        # which mean replace userIds value by source named "sellerIdList"
+        @argumentTransform(argumentName: "userIds", 
+            operateType: MAP, 
+            expression: "sellerIdList", 
+            dependencySources: ["sellerIdList"]
+        ){
+            userId
+            name
+            age
+        }
+}
+```
+
+See [graphql-calculator README](https://github.com/graphql-calculator/graphql-calculator) for more information.