Add API for pushing content entities to services as entities

https://wit.ai/docs/http/20170307#post--entities-link Yep, Wit.ai supports it.

So here's my wip.

Architecture is as follows:

• Config entity (EntityCollection) that defines a 'collection of entities we want to push somewhere'
• This includes an entity type id, and bundle field - so we say 'Collection' of 'Taxonomy terms' from 'Tags' bundle
• Also includes optional 'synonyms' field which will allow user to nominate a multi-value text field to derive synonyms from
• On it is a plugin collection 'query handlers' which is an array of plugins that 'find me some entities to push' which will use the entity type and bundle to query for entities. There is a base implementation that just filters by bundle and published if those keys exist for the given entity type. There is a deriver so that the user one can add the status field (which is not part of its annotation).
• On it is also a plugin collection 'push handlers' which is an array of plugins that 'push entities to an endpoint'
• I've got a test one that just saves them in state, so we can test the wiring.
• Have written one for api.ai that pushes there (see patch over there)
• The module has hook_entity_{insert|update|delete} implementations which just query for a matching collection and add it to a queue
• The a queue worker sends on cron, making sure to only send once - so multiple updates might queue the same collection multiple times, but we have a timestamp to check that

To do:

• in the config entity presave, push handlers should be able to react to a new entity (so e.g. api.ai can push a shell and get a remote id, then store that with the config entity for update sake)
• configuration form
• config dependencies (needs to depend on bundle if config based and field for synonyms)

done for today

to do:

- edit form submit handler
- make ui test pass (complete the form - TDD)
- config dependencies test
- config dependencies

Yep, the applies isn't yet implemented, but will be for the UI

i.e. limiting query handlers based on the entity type.

I'm going to do some test to better understand the UI and UX, but generally I'm happy with the work done so far.

The form still needs to be finished - that's what I'm working on now - hopefully it will make a bit more sense once that is done

This is ready for review now, working nicely on my local environment/api.ai account

Ah yeah - I changed the way the plugins were stored on the config entity, I filter out the disabled ones - so the isEnabled isn't needed.

Will remove on Monday

I didn't get to this hoping to get to it tomorrow

Removed the isEnabled method from the base class and the interface.

Added some additional information regarding cron etc to the messages/UI

Proper interdiff this time
And a fix for test groups

Committed and pushed to 8.x-1.x

Added @gambry in credits for all the reviews

Thanks! excited to get this in.

Will update project page to mention it.