[Infra Monitoring UI] use observable to watch for config changes

[neptunian]

As part of #140766, @miltonhultgren pointed out we could use an observable so when a user changes a setting, they don't have to refresh the app. Eg: https://github.com/elastic/kibana/blob/main/x-pack/plugins/security_solution/public/plugin.tsx#L491-L494

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)

[miltonhultgren]

Some notes before I start implementing (tomorrow):
To get the Observable of the UI setting we can do like we did here https://github.com/elastic/kibana/blob/main/x-pack/plugins/monitoring/public/application/index.tsx#L103-L107
The plugin file is a normal JS class, not a React functional component so we'll have to use the Observable directly, without hooks.

Then there are two places to update, the Kibana core deep links and the Observability sidebar links.

For the Kibana core deep links, we can add an updater$ to our application registration https://github.com/elastic/kibana/blob/main/packages/core/application/core-application-browser/src/application.ts#L148-L180, it's an Observable where you can feed changes to the application. If we subscribe to the UI setting observable and push new events to this updater$ stream it should update the deep links.

For the Observability side bar links, that register flow already uses an Observable that we today source from the start services but we can merge that stream with the UI settings to create new "sections" each time that setting (or the start services) change.