feat: Add slots to add tab links and add mechanism for plugin routes

[xitij2000]

Description

This PR adds mechanisms to add course tabs using a frontend plugin.

It includes two plugin slots:

• course_tab_links_slot: This slot allows adding additional course tabs or modifying existing course tabs.
• course_page_route_slot: This slot allows create a full new page in the learning MFE.

It also adds a new mechanism to add course routes using the PLUGIN_ROUTES config. This is a simple array that adds new routes for the MFE that can then be handled by plugins.

The combination of the above will allow injecting a new course tab by:

• Adding a new route to the PLUGIN_ROUTES array.
• Adding a slot to the course page route slot so that the page is rendered on that route
• Adding a new link to the course_tab_links_slot.

It is also possible to add the links anywhere else, to simply add new pages that are not tabs.

Other Information

Currently there is a single plugin slot for all pages and each plugin would need to check if the route matches and render itself. Another alternative is for the plugin routes to be a mapping of plugin name to the route, and the plugin name can be added to the slot ID. For example:

PLUGIN_ROUTES = {
  tab1: "/course/:courseId/tab1",
  tab2: "/course/:courseId/tab2",
};

<PluginSlot id={`${pluginName}_page_route_slot`} ... />

This would create slots like: tab1_page_route_slot and tab2_page_route_slot etc.

Or, it can just use the route itself in the slot name. While this will result in clumsy names like: "/course/:courseId/tab1_page_route_slot" there is not reason why that shouldn't work.

[openedx-webhooks]

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