[data views] fields_for_wildcard / fields REST apis can be somewhat slow, move processing to client

[mattkime]

A scalability test of the fields_for_wildcard and new fields REST api calls has shown that the processing of larger field_caps responses can be somewhat slow and introduce a fair amount of server load.

Attempts to make the code more efficient have either failed or are too small to make a meaningful difference. Instead, we could move the field_caps response processing to the client. This has the advantage of allowing the browser to skip the processing of the response if its a 304.

We haven't specifically seen any complaints regarding this but I still thought it was worth documenting.

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[kertal]

Closing this because it's not planned to be resolved in the foreseeable future. It will be tracked in our Icebox and will be re-opened if our priorities change. Feel free to re-open if you think it should be melted sooner.