[Actions][7.x] Move saved object fields to references for action_task_params

[chrisronline]

Relates to #100067

To prepare for the upcoming saved object ID migration, we need to move saved object fields to use references properly so the 8.0 migration will automatically update them.

We need to add a migration that will move action_task_params.relatedSavedObjects and action_task_params.actionId to saved object references. For both, there isn't exactly a 1-1 match between the data structures.

For actionId, I'd imagine the reference object to look like: { id: actionId, type: 'action', name: 'action' }

For relatedSavedObjects, it's a little more complicated. The id and type are straight-forward, but the saved object reference requires a name which might map to relatedSavedObject.typeId or something similar. The other piece of noteworthy data is relatedSavedObject.namespace which I think can safely be dropped, as the action_task_params saved object will always exist in the same space as the action that created it.

In addition to the migration, we need to change the code that writes and reads.

See the PoC PR for more insight into how this might work.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)