fix: Refactor CMSMenu class for easier subclassing

[fsbraun]

Description

The CMSMenu class has been updated in #7807 for efficient and versioning-compatible menus.
This PR factors out two major changes to allow for future subclassing (such as asked for in django-cms/djangocms-versioning#387):

• CMSMenu.get_pagecontent_queryset: returns the queryset to build the menu. The default implementation includes the admin-visible latest_content, e.g. not yet published pages, in edit and preview mode
• CMSMenu.preview_endpoint: returns a callable to get an endpoint for the menu entry. The default implementation returns the preview endpoint in edit or preview mode.

The PR does not change the logic of how CMSMenu works.

Related resources

• Bug (or feature?): CMSMenu.get_nodes() returns URLs of unpublished pages djangocms-versioning#387
• #...

Checklist

• I have opened this pull request against develop-4
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[fsbraun]

@jrief Do you think this refactor will help you customize the menu system?

[jrief]

@jrief Do you think this refactor will help you customize the menu system?

The bigger problem I have is that the menu renderer can't be replaced by a customized implementation, but that's a different story. I will merge this PR into my private branch and test it. Make take some days, though.

[fsbraun]

@jrief Is this still an issue? Or shall I close this?