[Deprecation] Collect deprecated configs usage

[Bamieh]

We need a way to collect the used deprecated configs in kibana. This is more challenging that I intially thought it would be since

Things to consider:

• We have no direct way of knowing these configs via config.deprecations since we allow plugins to freely mutate the whole config object. Potential solutions:
  • Use a diff of the previous object and the new object
  • Add an API in @kbn/config to allow the deprecationsHook to specify which keys change
    • Feels like the most practical solution at this time
  • Ideal solution would be for ConfigDeprecationFunction to return just the keys that changed rather than expose an entire mutable object

[elasticmachine]

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

[elasticmachine]

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