feat: add frontend-plugin-framework slots (#545)

Add the following `frontend-plugin-framework` slots:
* `logo_slot`
* `desktop_main_menu_slot`
* `desktop_secondary_menu_slot`
* `mobile_main_menu_slot`
* `course_info_slot`
* `learning_help_slot`
* `desktop_logged_out_items_slot`
* `mobile_logged_out_items_slot`
* `mobile_user_menu_slot`
* `desktop_user_menu_slot`
* `learning_user_menu_slot`
* `learning_logged_out_items_slot`
* `desktop_header_slot`

src/plugin-slots/DesktopHeaderSlot/README.md

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+# Desktop Header Slot
+
+### Slot ID: `desktop_header_slot`
+
+## Description
+
+This slot is used to replace/modify/hide the entire desktop header.
+
+## Examples
+
+### Custom Component
+
+The following `env.config.jsx` will replace the desktop header entirely (in this case with a centered 🗺️ `h1`)
+
+![Screenshot of custom component](./images/desktop_header_custom_component.png)
+
+```jsx
+import { DIRECT_PLUGIN, PLUGIN_OPERATIONS } from '@openedx/frontend-plugin-framework';
+
+const config = {
+  pluginSlots: {
+    desktop_header_slot: {
+      keepDefault: false,
+      plugins: [
+        {
+          op: PLUGIN_OPERATIONS.Insert,
+          widget: {
+            id: 'custom_desktop_header_component',
+            type: DIRECT_PLUGIN,
+            RenderWidget: () => (
+              <h1 style={{textAlign: 'center'}}>🗺️</h1>
+            ),
+          },
+        },
+      ]
+    }
+  },
+}
+
+export default config;
+```

src/plugin-slots/DesktopHeaderSlot/index.jsx

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+import React from 'react';
+import { PluginSlot } from '@openedx/frontend-plugin-framework';
+import DesktopHeader, { desktopHeaderDataShape } from '../../desktop-header/DesktopHeader';
+
+const DesktopHeaderSlot = ({
+  props,
+}) => (
+  <PluginSlot
+    id="desktop_header_slot"
+    slotOptions={{
+      mergeProps: true,
+    }}
+  >
+    <DesktopHeader {...props} />
+  </PluginSlot>
+);
+
+DesktopHeaderSlot.propTypes = desktopHeaderDataShape;
+
+export default DesktopHeaderSlot;