Support filter parameters in Configuration

Based on work done in doctrine/mongodb-odm#908

My understanding is that this makes it easier to setup filters at boot time, instead of having to fetch them from the collection later on.

For added context, the related PR from MongoDB ODM's Symfony bundle is  doctrine/DoctrineMongoDBBundle#255
I thought about doing that in the ODM PR; however, I saw that `getFilterClass()` was being used to check if a filter exists based on a `null` return value. I figured it was safer to mimic that behavior here as well.