Implements support for displaying imported Mimir/Prometheus time intervals
in Grafana following the templates pattern.
Changes:
- Add Kind field to MuteTimeInterval model to differentiate Grafana vs Mimir intervals
- Add includeImported support to MuteTimingService following templates pattern
- Update GetMuteTimings to read and expose imported intervals from ExtraConfigs
- Fix GetMuteTiming UID lookup to work with kind-prefixed UIDs
- Block create/update/delete operations on imported intervals (read-only)
- Wire up service with WithIncludeImported() in ngalert initialization
- Update dependency to alerting@c56cf15 which includes TimeIntervalKind
All changes follow the templates.go pattern.