[OneDiscover] Show ECS description in field popover

[kertal]

📓 Summary:

We already show data view field descriptions when set in data view mode.

It can be added in the data view edit mode

In the UnifiedFieldList

In the UnifiedDocViewer

Given there's no data view field description is set and there's ECS description data available for a field, the ECS description should be displayed. Since ES|QL mode doesn't use data views, it should just show the ECS description, if it's available.

✔️Acceptance criteria:

• In data view mode in Discover, when showing the field popover of the UnifiedFieldList or the UnifiedDocViewer, there's no custom data view field description for a given field set and ECS field description data is available, the ECS Description is displayed in the popover
• In ES|QL mode, the ECS description should be displayed, when it's available for a field.
• There's appropriate test coverage

[elasticmachine]

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