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/DesktopUserMenuSlot/README.md

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+# Desktop User Menu Slot
+
+### Slot ID: `desktop_user_menu_slot`
+
+## Description
+
+This slot is used to replace/modify/hide the desktop user menu.
+
+## Examples
+
+### Modify Items
+
+The following `env.config.jsx` will modify the items in the desktop user menu.
+
+![Screenshot of modified items](./images/desktop_user_menu_modify_items.png)
+
+```jsx
+import { PLUGIN_OPERATIONS } from '@openedx/frontend-plugin-framework';
+
+const modifyUserMenu = ( widget ) => {
+  widget.content.menu = [
+    {
+      items: [
+        {
+          type: 'item',
+          href: 'https://openedx.org/',
+          content: 'openedx.org',
+        },
+        {
+          type: 'item',
+          href: 'https://docs.openedx.org/en/latest/',
+          content: 'Documentation',
+        },
+      ]
+    },
+    {
+      items: [
+        {
+          type: 'item',
+          href: 'https://discuss.openedx.org/',
+          content: 'Forums',
+        }
+      ]
+    }
+  ];
+  return widget;
+};
+
+const config = {
+  pluginSlots: {
+    desktop_user_menu_slot: {
+      keepDefault: true,
+      plugins: [
+        {
+          op: PLUGIN_OPERATIONS.Modify,
+          widgetId: 'default_contents',
+          fn: modifyUserMenu,
+        },
+      ]
+    },
+  },
+}
+
+export default config;
+```
+
+### Replace Menu with Custom Component
+
+The following `env.config.jsx` will replace the desktop user menu entirely (in this case with a centered 🗺️ `h1`)
+
+![Screenshot of custom component](./images/desktop_user_menu_custom_component.png)
+
+```jsx
+import { DIRECT_PLUGIN, PLUGIN_OPERATIONS } from '@openedx/frontend-plugin-framework';
+
+const config = {
+  pluginSlots: {
+    desktop_user_menu_slot: {
+      keepDefault: false,
+      plugins: [
+        {
+          op: PLUGIN_OPERATIONS.Insert,
+          widget: {
+            id: 'custom_user_menu_component',
+            type: DIRECT_PLUGIN,
+            RenderWidget: () => (
+              <h1 style={{textAlign: 'center'}}>🗺️</h1>
+            ),
+          },
+        },
+      ]
+    },
+  },
+}
+
+export default config;
+```
+
+### Add Custom Components before and after Menu
+
+The following `env.config.jsx` will place custom components before and after the desktop user menu (in this case centered `h1`s with 🌞 and 🌚).
+
+![Screenshot of custom components before and after](./images/desktop_user_menu_custom_components_before_after.png)
+
+```jsx
+import { DIRECT_PLUGIN, PLUGIN_OPERATIONS } from '@openedx/frontend-plugin-framework';
+
+const config = {
+  pluginSlots: {
+    desktop_user_menu_slot: {
+      keepDefault: true,
+      plugins: [
+        {
+          op: PLUGIN_OPERATIONS.Insert,
+          widget: {
+            id: 'custom_before_user_menu_component',
+            type: DIRECT_PLUGIN,
+            priority: 10,
+            RenderWidget: () => (
+              <h1 style={{textAlign: 'center'}}>🌞</h1>
+            ),
+          },
+        },
+        {
+          op: PLUGIN_OPERATIONS.Insert,
+          widget: {
+            id: 'custom_after_user_menu_component',
+            type: DIRECT_PLUGIN,
+            priority: 90,
+            RenderWidget: () => (
+              <h1 style={{textAlign: 'center'}}>🌚</h1>
+            ),
+          },
+        },
+      ]
+    },
+  },
+}
+
+export default config;
+```

src/plugin-slots/DesktopUserMenuSlot/index.jsx

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+import React from 'react';
+import { PluginSlot } from '@openedx/frontend-plugin-framework';
+import DesktopHeaderUserMenu, { desktopUserMenuDataShape } from '../../desktop-header/DesktopHeaderUserMenu';
+
+const DesktopUserMenuSlot = ({
+  menu,
+}) => (
+  <PluginSlot
+    id="desktop_user_menu_slot"
+    slotOptions={{
+      mergeProps: true,
+    }}
+  >
+    <DesktopHeaderUserMenu menu={menu} />
+  </PluginSlot>
+);
+
+DesktopUserMenuSlot.propTypes = {
+  menu: desktopUserMenuDataShape,
+};
+
+export default DesktopUserMenuSlot;