[Discover] Add customization point to customize the unified list field popover

[logeekal]

Describe the feature:

Currently, discover exposes a customization Fwk which lets discover clients customize various parts of discover.

While going through the Discover-Timeline integration project, I noticed that currently, there is not way to customize the discover popover in unified field list.

This issue created to request that particular customization point as currently, we would like to hide visualize icon as we do not want users to redirect to Lens app from security solution in serverless environment.

Describe a specific use case for the feature:

☝🏼Described above.

[elasticmachine]

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