Add limited support for ExternalBugs extension in RHBZ

- RHBZ instances now support adding/removing external trackers as
  documented at https://bugzilla.redhat.com/docs/en/html/api/extensions/ExternalBugs/lib/WebService.html
- Limitations: current implementation uses only ext_type_description and
  ext_bz_bug_id and does not support any other external tracker fields