Closes #178742

## Summary

In the old embeddable system, when an embeddable is created through its
factory, it is "enhanced" via `enhanceEmbeddableWithDynamicActions` in
`EmbeddableEnhancedPlugin` - this enhancement is responsible for adding
the pieces necessary (for example, the `DynamicActionManager`) to the
embeddable to manage drilldowns. In the new system, we are no longer
using these class-based factories - so there is no longer a way to
enhance embeddables on creation. Therefore, this PR does three main
things:

1. It adds a new method `initializeDynamicActions` to the
`EnhancedEmbeddablePlugin` and returns it through its start contract (in
order to bypass limitations around importing stuff from `xpack`) - this
method can be used to enhance all **React-based** embeddables with the
stuff necessary for drilldowns to work. As part of this, I had to add a
few extra pieces to the `HasDynamicActions` API:
- `dynamicActionsState$`: This is a publishing subject that will keep
track of the events currently attached to the given embeddable
(`DynamicActionsState`)
     - `setDynamicActions`: A setter for the above publishing subject
2. It modifies all actions tied to dashboard drilldowns -
`PanelNotificationsAction`, `EmbeddableToDashboardDrilldown`,
`FlyoutEditDrilldownAction`, and `FlyoutCreateDrilldownAction`.
3. It modifies the old `enhanceEmbeddableWithDynamicActions` to add the
publishing subject + setter from (1) above to all **legacy** embeddables
when they are enhanced - that way, they will still pass the
`apiHasDynamicActions` type check + all modified actions continue to
work for legacy embeddables.

**How to Test**
To test with a React embeddable, you can checkout the [draft
PR](#178544) containing the new
image embeddable.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios



### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

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Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>