feat(handler): Accept a GraphQL execution rootValue

Closes #30

docs/interfaces/handler.HandlerOptions.md

@@ -22,6 +22,7 @@
 - [onOperation](handler.HandlerOptions.md#onoperation)
 - [onSubscribe](handler.HandlerOptions.md#onsubscribe)
 - [parse](handler.HandlerOptions.md#parse)
+- [rootValue](handler.HandlerOptions.md#rootvalue)
 - [schema](handler.HandlerOptions.md#schema)
 - [validate](handler.HandlerOptions.md#validate)
 
@@ -197,6 +198,19 @@ Throws GraphQLError if a syntax error is encountered.
 
 ___
 
+### rootValue
+
+• `Optional` **rootValue**: `unknown`
+
+The GraphQL root value or resolvers to go alongside the execution.
+Learn more about them here: https://graphql.org/learn/execution/#root-fields-resolvers.
+
+If you return from `onSubscribe`, and the returned value is
+missing the `rootValue` field, the relevant operation root
+will be used instead.
+
+___
+
 ### schema
 
 • `Optional` **schema**: `GraphQLSchema` \| (`req`: [`Request`](handler.Request.md)<`RequestRaw`, `RequestContext`\>, `args`: `Omit`<[`OperationArgs`](../modules/handler.md#operationargs)<`Context`\>, ``"schema"``\>) => [`Response`](../modules/handler.md#response) \| `GraphQLSchema` \| `Promise`<[`Response`](../modules/handler.md#response) \| `GraphQLSchema`\>

src/handler.ts

@@ -215,6 +215,15 @@ export interface HandlerOptions<
    * GraphQL operation AST getter used for detecting the operation type.
    */
   getOperationAST?: typeof graphqlGetOperationAST;
+  /**
+   * The GraphQL root value or resolvers to go alongside the execution.
+   * Learn more about them here: https://graphql.org/learn/execution/#root-fields-resolvers.
+   *
+   * If you return from `onSubscribe`, and the returned value is
+   * missing the `rootValue` field, the relevant operation root
+   * will be used instead.
+   */
+  rootValue?: unknown;
   /**
    * The subscribe callback executed right after processing the request
    * before proceeding with the GraphQL operation execution.
@@ -366,6 +375,7 @@ export function createHandler<
     execute = graphqlExecute,
     parse = graphqlParse,
     getOperationAST = graphqlGetOperationAST,
+    rootValue,
     onSubscribe,
     onOperation,
   } = options;
@@ -576,6 +586,10 @@ export function createHandler<
       ];
     }
 
+    if (!('rootValue' in args)) {
+      args.rootValue = rootValue;
+    }
+
     if (!('contextValue' in args)) {
       const resOrContext =
         typeof context === 'function' ? await context(req, params) : context;