Add fields standard query parameter to Job and use it in FileFetchJob.

Summary:
 Add `fields` standard query parameter to Job and use it in FileFetchJob. This change requires clients using this library to change the way they limit what fields are returned in a FileFetchJob.
Start adding entity field declarations for use with Job::setFields().

See [[ https://phabricator.kde.org/D19948 | D19948 DRAFT Add Standard Query Parameters support ]] for previous discussions.

Reviewers: dvratil

Subscribers: elvisangelaccio, kde-pim, #libkgapi

Tags: #kde_pim, #libkgapi

Differential Revision: https://phabricator.kde.org/D20886