Introduces new incremental response format (#3886)

The `incremental` array is the actual data to be applied to the response, while the `completed` array return "metadata" about the execution, used to inform clients that defers are being executed and when all fields for that defer have been delivered.

To ensure consistency, clients are expected to process all objects in the `incremental` array for a given payload before re-rendering the associated UIs.

See graphql/defer-stream-wg#69 for full details.

src/execution/__tests__/mutations-test.ts

@@ -242,13 +242,13 @@ describe('Execute: Handles mutation execution ordering', () => {
       {
         incremental: [
           {
-            label: 'defer-label',
             path: ['first'],
             data: {
               promiseToGetTheNumber: 2,
             },
           },
         ],
+        completed: [{ path: ['first'], label: 'defer-label' }],
         hasNext: false,
       },
     ]);
@@ -317,7 +317,6 @@ describe('Execute: Handles mutation execution ordering', () => {
       {
         incremental: [
           {
-            label: 'defer-label',
             path: [],
             data: {
               first: {
@@ -326,6 +325,7 @@ describe('Execute: Handles mutation execution ordering', () => {
             },
           },
         ],
+        completed: [{ path: [], label: 'defer-label' }],
         hasNext: false,
       },
     ]);