[Telemetry] Move all collection logic to Stats/Usage collectors

[afharo]

At the moment, when building the Telemetry payload, on top of retrieving all the Stats and Usage collectors' data, the telemetry plugin appends some data to it. Examples of the latter are Data Telemetry, License data and ES's cluster_stats, Nodes Usage + X-Pack usage.

This issue is to discuss if it's worth moving this logic to their own collectors. IMO, this will help with maintaining the full schema (#83704), and those sources of information can be reused, if needed, by the Suggestion Engine, since they can be called as any other usage/stat collector.

While implementing this, we might want to explore a new API that allows collectors to specify the absolute path where they want to be merged in as needed in #89110.

What do you think?

[elasticmachine]

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

[Bamieh]

I think we'd need a small POC for this to decide. The implementation details are the tricky part here which might restrict our options.