[UnifiedFieldList] Reduce requests for data view fields

[kertal]

To show the Unified Field List it currently takes 2 request for the fields that are part of the currently selected DataView.

• The first request is fetching all fields, this happens when a data view is being loaded/constructed
• The second request is aiming to get the fields that match the given query, filters (including the time range), this is used to split between available and empty fields.

With the implementation of #167750 is is possible to work with DataViews without automatic field fetching. Having this available would unlock the following potential improvement:

• We could use the new class to load just the fields matching the current query & filter, these would be the available fields

• When users would request to see the "Empty fields", this could trigger a request to fetch all fields, and the previously available fields could be used to split between Empty fields and Available fields

This change would lead to better performance especially in large installations. In an investigation, the request for all fields took 8s, a request for "Available fields" 2s, which would be significant performance improvement

tbc.

[elasticmachine]

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

[thomasneirynck]

There actually seems to be three requests. Two identical ones (without the filter), followed by one with the filter.

This is a bug I think, and should track separately. #167885

[kertal]

@thomasneirynck you're right, but it's already tracked in:
#167221