Specify OQL Support

## OpenProject work packages

* https://community.openproject.org/work_packages/17426
I would have gone as far as specifying that all attributes listed are eligable for filtering.

In the dire hope that an implementation could be generic enough to support that "for free".

Otherwise I would fear that we fail to keep track of all those exceptions (you can do a, but not for b, and you can do c, but not for d)