[Navigation-Next] Support nested items in left navigation.

[SuZhou-Joe]

Is your feature request related to a problem? Please describe.

In the new navigation, we are seeking for a solution to enable nested items for some sub applications within plugins. For the example above, the detection rules and log types are the sub applications we are talking about.

Describe the solution you'd like

Add a new field in ChromeRegistrationNavLink:

export interface ChromeRegistrationNavLink {
  ...,
  parentNavLinkId?: string;
};

core.chrome.addNavLinksToGroup(DEFAULT_NAV_GROUPS.observability, [{ id: 'detectionRule', parentNavLinkId: 'threatDetectors' }]);

And in such case, we are able to support infinite levels of nested menu items if we want.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

#7027

[SuZhou-Joe]

Has been supported in #7060