[Lens] Cache field existence requests

[flash1293]

Thanks to the client side request cache which is part of the search service, requests for main chart and suggestions are cached when going back and forth between Lens and Dashboard.

However before Lens becomes usable, the field list has to render which blocks on the field existence request. This request is often pretty fast, but depending on the system it can take up to 5 seconds which can be a significant amount of time until the UI becomes usable.

We should apply a similar caching bound to the current session id for these requests to speed up the user flow.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[flash1293]

Related to #74493

[flash1293]

This isn't necessary anymore on the Lens side once #112782 is implemented