Support custom container extractors with the ORM mapper
Description
At the moment custom container extractors can be used with the ORM mapper, but the corresponding properties must have automatic indexing disabled (@IndexingDependency(reindexOnUpdate = ReindexOnUpdate.NO)). This is because the code translating container extractor path to paths in the ORM metamodel only understands some built-in extractors, and will fail on custom extractors.
We will need something more than the current ContainerExtractor interface in order to make the ORM mapper understand what a custom extractor "means" exactly.
To test it, we can use a custom container type with a corresponding UserCollectionType.
While we're at it, we should probably test automatic indexing for Optional, OptionalInt, arrays, etc. I think we only have tests for List, Map, Set at the moment.