Add pathBuilder argument to @rest directive

pathBuilder is a user provided function via a graphql variable to build up the path string. A use case it enables is having one or more optional query string parameters e.g:
"/items/filter?minPrice=5&maxPrice=20"
Here minPrice and maxPrice could be omitted

See proposal in #69 and the problem it solves in #67

src/__tests__/restLink.ts

@@ -772,6 +772,83 @@ describe('Query single call', () => {
       post: { ...postWithNest, __typename: 'Post' },
     });
   });
+
+  it('can build the path using pathBuilder', async () => {
+    expect.assertions(1);
+
+    const link = new RestLink({ uri: '/api' });
+    const posts = [{ id: '1', title: 'Love apollo' }];
+    fetchMock.get('/api/posts?status=published', posts);
+
+    const postTitleQuery = gql`
+      query postTitle($pathFunction: any, $status: String) {
+        posts(status: $status) @rest(type: "Post", pathBuilder: $pathFunction) {
+          id
+          title
+        }
+      }
+    `;
+
+    function createPath(variables) {
+      const qs = Object.keys(
+        variables,
+      ).reduce((acc: string, key: string): string => {
+        if (variables[key] === null || variables[key] === undefined) {
+          return acc;
+        }
+        if (acc === '') {
+          return '?' + key + '=' + String(variables[key]);
+        }
+        return acc + '&' + key + '=' + String(variables[key]);
+      }, '');
+      return '/posts' + qs;
+    }
+
+    const { data } = await makePromise<Result>(
+      execute(link, {
+        operationName: 'postTitle',
+        query: postTitleQuery,
+        variables: {
+          status: 'published',
+          pathFunction: createPath,
+        },
+      }),
+    );
+
+    expect(data).toMatchObject({
+      posts: [{ ...posts[0], __typename: 'Post' }],
+    });
+  });
+
+  it('throws if missing both path and pathBuilder', async () => {
+    expect.assertions(1);
+
+    const link = new RestLink({ uri: '/api' });
+    const post = { id: '1', title: 'Love apollo' };
+    fetchMock.get('/api/post/1', post);
+
+    const postTitleQuery = gql`
+      query postTitle {
+        post @rest(type: "Post") {
+          id
+          title
+        }
+      }
+    `;
+
+    try {
+      await makePromise<Result>(
+        execute(link, {
+          operationName: 'postTitle',
+          query: postTitleQuery,
+        }),
+      );
+    } catch (error) {
+      expect(error.message).toBe(
+        'Parmeter "path" or "pathBuilder" must be set in @rest directive',
+      );
+    }
+  });
 });
 
 describe('Query multiple calls', () => {

src/restLink.ts

@@ -107,12 +107,18 @@ export namespace RestLink {
     /** What GraphQL type to name the response */
     type?: string;
     /** What path to use */
-    path: string;
+    path?: string;
     /**
      * What endpoint to select from the map of endpoints available to this link.
      * @default `RestLink.endpoints[DEFAULT_ENDPOINT_KEY]`
      */
     endpoint?: string;
+    /**
+     * Optional function that constructs a request path out of the Environmental state
+     * when processing this @rest(...) call. 
+     * @default function that produces a request path string from the args.
+     */
+    pathBuilder?: (args: object) => string;
     /**
      * Optional method that constructs a RequestBody out of the Environmental state
      * when processing this @rest(...) call. 
@@ -387,19 +393,36 @@ const resolver: Resolver = async (
     typePatcher,
     fieldNameDenormalizer: linkLevelNameDenormalizer,
   } = context;
-  const { path, endpoint } = directives.rest as RestLink.DirectiveOptions;
+  let {
+    path,
+    endpoint,
+    pathBuilder,
+  } = directives.rest as RestLink.DirectiveOptions;
   const uri = getURIFromEndpoints(endpoints, endpoint);
   try {
     const argsWithExport = { ...args, ...exportVariables };
-    let pathWithParams = Object.keys(argsWithExport).reduce(
-      (acc, e) => replaceParam(acc, e, argsWithExport[e]),
-      path,
-    );
-    if (pathWithParams.includes(':')) {
-      throw new Error(
-        'Missing params to run query, specify it in the query params or use an export directive',
-      );
+
+    if (!pathBuilder) {
+      if (!path) {
+        throw new Error(
+          'Parmeter "path" or "pathBuilder" must be set in @rest directive',
+        );
+      }
+      pathBuilder = (args: object): string => {
+        const pathWithParams = Object.keys(argsWithExport).reduce(
+          (acc, e) => replaceParam(acc, e, argsWithExport[e]),
+          path,
+        );
+        if (pathWithParams.includes(':')) {
+          throw new Error(
+            'Missing params to run query, specify it in the query params or use an export directive',
+          );
+        }
+        return pathWithParams;
+      };
     }
+    const pathWithParams = pathBuilder(argsWithExport);
+
     let {
       method,
       type,