[ObservabilityAIAssistant] Optimize initial load bundles

[tonyghiani]

📓 Summary

Part of #191599

The Observability AI Assistant App plugin loads an icon in the navbar to access the AI assistant. The issue, in this case, is that, even if the icon is not visible for a specific app, the visibility check occurs too late in the react tree, which implies loading all the code used for the AIAssistant control even when this is not visible for the user.

This can be potentially fixed by moving the assertion earlier in the rendering step, and applying correctly the lazy loading on the LazyNavControl component.

• LazyNavControl usage.
• Late conditional render.

[elasticmachine]

Pinging @elastic/obs-ux-logs-team (Team:obs-ux-logs)