[Usage Collection] Application Usage automatic schema

[afharo]

This is an improvement over #70622.

In that issue, we added the usageCollection.schema option to help with the automation of the schema validation for every release.

This issue is to track the creation of some sort of automated process to update that schema with all the keys retrieved by checking all the requests to core.application.register and building up the schema dynamically.

[elasticmachine]

Pinging @elastic/kibana-telemetry (Team:KibanaTelemetry)

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[afharo]

Ideally, we can move to an array format and simplify the schema a lot.

Another possibility is to provide the DYNAMIC_KEY keyword we previously agreed on with @mindbat.

Finally, we can run some functional tests that list all the apps in Kibana and check they are listed in the schema. The problem with this last approach is that it will depend on whether plugins are enabled/disabled while running the tests.

[afharo]

We can keep updating manually but add FTR that retrieves the list of apps from Core and checks the schema is up to date.