Allowing associations in Query::contain to specify a custom finder

This allows changing the finder method an associated Table uses for contains. Also, `/ORM/Association/SelectableAssociationTrait` was passing in "all" at all times to the Query, even if you changed the finder. This addresses #4408.

Thanks!
Sorry about this one. The test that was in there was testing the same thing - finder options. The change I made to this makes it more readable and is more in line with this PR as a whole.