[Observability] Add Observability Shared app to house shared components

[CoenWarmer]

Part of #152783

📝 Summary

The problem
The UX app uses an Embeddable via the Exploratory View app on the Observability Overview page.
Therefore the Observability App requires the Exploratory View app.

The Exploratory View app needs to use the same Page Template as the Observability app in order to keep the same navigation structure as the rest of the Observability apps.

This relationship creates a cyclical dependency between these two apps. No bueno!

Potential solutions
There are three ways to solve this problem (perhaps more?):

1. Have Synthetics team create a Overview page widget which does not use the Exploratory View app, so the Observability app does not need to depend on the Exploratory View app;
2. Consolidate all Observability app inside one package so there are no ways for cyclical dependencies to occur;
3. Create a separate app that houses components which are shared by all Observability apps, which does not depend on any of the Observability apps, so all downstream apps can safely depend on it.

✅ Acceptance criteria

• There should be no cyclical dependencies between the Observability app and the Exploratory View app
• The UX team should not have to recreate the Observability Overview widget
• All Observability apps should be able to use the same Observability Page Template

[elasticmachine]

Pinging @elastic/actionable-observability (Team: Actionable Observability)