Expose indexing service, add command

[tacman]

Since you've already implemented calling the indexer and creating indexes for Behat, can you expose the service that does the index and perhaps add a Symfony command that demonstrates how to call / use it?

An injectable service would allow indexing to easily called from a web page or background message processor (e.g. using rabbitmq).

A command line demonstration would allow developers to customize their cron jobs and other tools. For example, I have one script that calls the indexer that checks the timestamp of the .spp file against the most recent database update of the relevant tables to determine if indexing is needed. I'd love to be able to call an indexing service rather than exec'ing indexer.

Perhaps you already offer some of this, in which case it's just a matter of documenting how to inject the service and trigger the indexing. I'm using Symfony4, so an example that didn't use the container would be very helpful.

Thanks.