feat: add filter affected services internal endpoint

[matiasb]

Related to https://github.com/grafana/oncall-private/issues/2977

e.g.

GET /api/plugins/grafana-oncall-app/resources/alertgroups/filter_affected_services?service=service-a&service=service-b

[
  {"name": "service-a",
   "service_url": "http://localhost:3000/a/grafana-slo-app/service/service-a",
   "alert_groups_url": "http://localhost:3000/a/grafana-oncall-app/alert-groups?status=0&status=1&started_at=now-7d_now&label=service_name:service-a"}
]