[Lens] Improve field existence check data fetching

[flash1293]

See #70520

The main question here is whether we can stop making three requests for field existence, specifically the mapping request. The mapping request is used to determine that .keyword fields are multi-mapped, and we might have two ways of avoiding the need for this request:

1. We could use the new fields request parameter, which is partially available: Search 'fields' option design + implementation elasticsearch#55363. This new API would let us skip the mapping and field_caps requests.

2. We could skip the mapping query by adding the docvalue_fields parameter to the 500 docs search request for certain fields. There is a 100 field limit to this approach, so I think we probably can't use this in all cases.

The new fields request parameter was introduced mostly to solve this problem

[elasticmachine]

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

[wylieconlon]

The new fields parameter is still in a feature branch, but based on recent discussions with the ES team it sounds like it will be merged soon. I think we should hold off on making any changes until it's ready, since it's the best option.