Generate MenuItems api for application menu generation

[caalador]

Describe your motivation

There exists the MenuRegistry that can be used for collecting menu item and route data, but the implementation is aimed for internal consumption and is not as user friendly as it could be.

Describe the solution you'd like

There should be a MenuConfiguration class that returns MenuOptions objects as an ordered collection for use within applications.

Describe alternatives you've considered

Update and remake the current MenuRegistry api.

[mshabarov]

Acceptance Criteria (draft)

• MenuRegistry is moved under com.vaadin.flow.internal.menu and/or documented as "for internal use only".
• If moved to internal package, Hilla and Start are modified accordingly.
• MenuConfiguration new class is now public class that has needed methods for dynamic menus, similar to RouteConfiguration class that is public API.
• MenuConfiguration has methods getAvailableMenuOptions (or similar name) + getAvailableClientMenuOptions that returns new MenuOptions class (or as an alternative, the existing MenuData class? )
• MenuOptions can give a collection or stream of MenuOption
• MenuOption class has path, title, order, exclude and icon fields (what else is needed?)
• Online documentation is updated to use MenuConfiguration class.

[mshabarov]

Should include fixes for:

• Inconsistency in MenuData.icon between Flow and Hilla views without icon #20114
• AvailableViewInfo.menu is null if a Hilla route has no menu in its view config #20109

[knoobie]

MenuOption class has path, title, order, exclude and icon fields (what else is needed?)

Should this include a reference to the class from which the menu comes from? Custom Menu Annotations are probably a bit hard to support.. but this would at least allow to gather all other annotation / informations from the Menu's class.

[mshabarov]

Sounds like a useful data to have there, agreed.