[Discover][Extension] Default state (columns - incl. width, rowHeight)

[kertal]

📓 Summary

Depending on the context Discover should be able to provide good defaults depending on the data source that's selected. We are aiming to use default columns (including default column width), default row height, and more to the user when the data source is selected / entered initially. Internally we refer to those values as a subset of Discover's state, which is fragmented in 5 state containers, so it makes sense not to make it configureable in a granular but in a high level way.

✔️ Acceptance criteria

• Provide an extension point in the contextual awareness architecture allowing to provide default values for the following properties:
  • columns (array of columns displayed in the data table, part of DiscoverAppState, columns equals fieldnames)
    • https://github.com/elastic/kibana/blob/872f01064873133cc53485f007a0ec9cff6adf84/src/plugins/discover/public/application/main/state_management/discover_app_state_container.ts#L94-L97
  • grid.columns (this is an object of data table columns by name, having a width parameter for columns where the width differs from the default one)
    • https://github.com/elastic/kibana/blob/872f01064873133cc53485f007a0ec9cff6adf84/src/plugins/discover/public/application/main/state_management/discover_app_state_container.ts#L102-L105
  • rowHeight (displayed row height of the data table)
    • https://github.com/elastic/kibana/blob/872f01064873133cc53485f007a0ec9cff6adf84/src/plugins/discover/public/application/main/state_management/discover_app_state_container.ts#L138-L141

[elasticmachine]

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