Add Rspamd support to KMail antispam

Summary:
This patch adds rspamd to the list of known antispam-tools, and thus enables KMail to display spam probability as assessed by Rspamd.

The support is limited to the evaluation of existing headers, as (to my knowledge) Rspamd can only be run server-side, and has no tools for analyzing offline mailboxes.

Reviewers: #kde_pim_kmail, dvratil

Reviewed By: dvratil

Subscribers: dvratil, #kde_pim

Tags: #kde_pim, #kde_pim_kmail

Differential Revision: https://phabricator.kde.org/D7751