Feature: Support selecting the top level of a Custom API response

Some APIs return arrays at the top. With this change, a customapi widget
can select the top level object/array.

This is particularly useful when combined with the `size` formatter, but
can also be used for APIs that return scalar values directly.

docs/widgets/services/customapi.md

@@ -57,6 +57,9 @@ widget:
     - field: key
       label: Number of things in array
       format: size
+    - field: . # This will take the root of the API response, e.g. when APIs return an array
+      label: Number of items
+      format: size
 ```
 
 Supported formats for the values are `text`, `number`, `float`, `percent`, `bytes`, `bitrate`, `size`, `date` and `relativeDate`.

src/widgets/customapi/component.jsx

@@ -5,9 +5,9 @@ import Block from "components/services/widget/block";
 import useWidgetAPI from "utils/proxy/use-widget-api";
 
 function getLength(data) {
-  if ('length' in data) {
+  if ("length" in data) {
     return data.length;
-  } else if (typeof data === 'object' && data !== null) {
+  } else if (typeof data === "object" && data !== null) {
     return Object.keys(data).length;
   } else {
     return NaN;
@@ -19,6 +19,11 @@ function getValue(field, data) {
   let lastField = field;
   let key = "";
 
+  // Support APIs that return arrays or scalars directly.
+  if (field === ".") {
+    return value;
+  }
+
   while (typeof lastField === "object") {
     key = Object.keys(lastField)[0] ?? null;