Parents processor for search_api

I'm working on the initial MR.

I've added the initial processor. Need to add tests. To use

1. Create your content type, with EH field, and a couple of nodes etc.
2. In your index, go to Processors tab, enabled "Parents" processor
3. In the configuration of the processor, confirm your EH field name, (defaults to `parents` but you might have `field_parents` for example)
4. In your Fields, in the General area, add Parents property. Give it a weird label because it's easier to find in Views etc.
5. Index
6. Make a view, add field, filter Search category, find your weirdly named field from above.
7. You should see `nid, nid, nid` all parents including self, and empty if it has no children/parents. If the top level node is 9, you should see 9
8. Add filter, set to a nid, should filter accordingly

Since you have to add the property in the Fields screen, could getPropertyDefinitions() find all the EH fields (usually there is only one) and offer them all as properties and thus be zero configuration?

or could use a plugin deriver

Dealer's choice, I can do that - i assume there's rarely more than one field (but assumptions assumptions...).

I believe it should also be abstracted to cover any entity type, which also makes the deriver more practical?

I'm not sure if/when i can continue this as the project I was about to use it on has been deferred. Just un-assigning for now.