* Introduce new models RoutingTree, RouteDefaults and Route and api-server to serve them that is backed by provisioning notification policy service.
* update method UpdatePolicyTree of notification policy service to return route and new version
* declare new actions alert.notifications.routes:read and alert.notifications.routes:write and two corresponding fixed roles.
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Co-authored-by: Tom Ratcliffe <tom.ratcliffe@grafana.com>
Co-authored-by: Matthew Jacobson <matthew.jacobson@grafana.com>
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