Specify Custom Fields

## OpenProject Work Package

TBD

## Progress

* [x] Specify schema as own resource
* [ ] Specify endpoint for work package schema
* [ ] Extend schema specification to suffice for reading purposes (including custom fields)
* [ ] Extend work package specification on how additional (non core) fields are represented

## Changes to existing implementation

* form includes link to schema resource
* schema resource promoted to a real type (with own endpoint, self-link, etc.)