Remove Layout Builder dependency for the config and rendering of the Navigation sidebar

Adding some related issues from the XB issue queue.

I've created a MR that removes the dependency on layout_builder while keeping the same config format for navigation.block_layout. The dependency is removed by simply calling the Layout and Block plugin managers directly.

The UI is still to be replaced. I only emptied navigation LayoutForm and added a new route "navigation.layout" to replace "layout_builder.navigation.view".

This currently moves down the dependency to layout_discovery. Is that a module that stays around in core? (There are a lot more criteria of course, but it is also not depended on by Experience Builder or UI Suite that I could find at least).

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

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@gábor hojtsy layout_discovery provides the layout plugin manager service definition.

📌 Add support for layout plugins Active doesn't depend on the layout_discovery service but it does depend on the same layout plugin manager class that the service uses (which is in Drupal\Core).

So I think that even if we eventually deprecate the layout_discovery module, the layout plugin manager itself will need to exist for quite a long time to support the just in time layout builder -> xb upgrade path. That could mean that navigation module ends up adding it's own version of the layout plugin manager or something like that - but it should be a relatively small incremental step to remove that dependency if/when we get to that point.