[Lens] Allow custom legend series actions / drilldowns

[ghudgins]

Describe the feature:
Allow a user to configure a drilldown-style action from a legend series taking the selection into another part of Elastic (like a Dashboard)

Describe a specific use case for the feature:
Analytics Author
When I am authoring an XY visualization with a breakdown by (and legend)
I need to add a custom action to the legend
So I can take an entire "series" into a tailor-made Dashboard for analyzing that series

Solution Developer
When I am composing my solution and using Lens Embeddable
I need to add a custom action to the legend
So I can take an entire "series" into another part of the solution focused on analyzing that series

Side note: I'm writing this issue as a follow-up to a discussion had with @stephmilovic who was very excited that @stratoula had added the filter series options to legends

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[flash1293]

Current drilldowns are defined on a filter, so the user has to first make a choice whether they want to filter "for value" or "out value" - one approach would be to make this menu nested - e.g if the user clicks "Filter for value", they will get a new menu with the drilldowns for that. If we want to show the drilldowns directly in that menu, we would need to decide whether we want to filter "for value" or "out value"

[stratoula]

Closing as duplicate of #90724