[WIP] Add parent_id to list of possible filters

This allows filtering by parent id using API v2 and possiblly other means using the `Query` class.

https://www.openproject.org/work_packages/15754

@ulferts I don't have too much of an idea of which list of filters means what. There's one in `query.rb`, one in `queries/work_packages/available_filter_options` and one in `queries/work_packages/filter.rb`.
Note also the diescrepancy between the `parent_id` field I added and the `parent` field still listed in `query.rb`. So we should discuss this before merging this.