[data.search.aggs] Support diversified sampler in AggConfigs #67388

Part of #60126
Related to #25983 - might be worth tackling both at once

This issue tracks support for diversified sampler aggregations in AggConfigs. Adding this agg type will unblock apps like Visualize and Lens from supporting these aggs in the future.

• add new agg type
  • create the agg type
  • export an interface for the agg type's params
  • update AggParamsMapping in types.ts
  • register agg type via agg_types.ts
  • unit tests
• add agg type expression function
  • write the expression function
  • register expression function via agg_types.ts
  • unit tests