[data.search.aggs] Support significant text in AggConfigs

[lukeelmers]

Part of #60126
Blocks #31614

This issue tracks support for significant text 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

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)