feat(GraphQL): Webhooks on add/update/delete mutations (GRAPHQL-1045) (…

…#7494) (#7616)

Please see the [RFC](https://discuss.dgraph.io/t/webhook-lambda-on-add-update-delete-mutations/12904) for more details.

(cherry picked from commit 348119e)

# Conflicts:
#	graphql/schema/testdata/schemagen/output/interface-with-id-directive-and-ID-field.graphql

edgraph/graphql.go

@@ -54,7 +54,7 @@ func ProcessPersistedQuery(ctx context.Context, gqlReq *schema.Request) error {
 		if err != nil {
 			return err
 		}
-		if _, err := validateToken(accessJwt[0]); err != nil {
+		if _, err := validateToken(accessJwt); err != nil {
 			return err
 		}
 	}