implement parts of the bug field api endpoint

Summary:
excessively unnecessary properties are not mapped currently as the only
use we have for this endpoint is to get the list of remotely known
platforms/distros. this may change in the future of course.

Test Plan: unit test passes

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22491