Remote Alertmanager: Optionally merge remote state before starting the internal Alertmanager (#107710)

* Remote Alertmanager: Use the same struct for Grafana stat and Mimir full state

* Alertmanager: Add methods to merge nflog and silences

* update grafana/alerting version

* make update-workspace

* update mocks

* remove unnecesary methods from the remote Alertmanager implementation, create separate StateMerger interface

* (WIP) Remote Alertmanager: Optionally merge remote state before starting the internal Alertmanager

* cleanup ngalert.go

* restore defaults.ini

* move state parsing logic to 'remote' package, clean up ngalert.go

* remove GetBase, implement MegeNflog and MergeSilences

* delete fmt.Println

* FetchRemoteState -> GetRemoteState

* UserGrafanaState -> UserState

* remove duplicate clusterpb import

* reorder MimirClient interface

* use general getState() method for Grafana state and Mimir full state

* remove unnecessary state merging methods from the Alertmanager interface

* remove pullState field

* reduce diff

* add info log after merging

* merge silences and nflog entries in the same method

* merge the remote state in the forked AM

* reduce diff

* update remote AM mock

* tests

* make error more specific

* typo

pkg/services/featuremgmt/toggles_gen.csv

@@ -240,3 +240,4 @@ alertingNotificationHistory,experimental,@grafana/alerting-squad,false,false,fal
 pluginAssetProvider,experimental,@grafana/plugins-platform-backend,false,true,false
 unifiedStorageSearchDualReaderEnabled,experimental,@grafana/search-and-storage,false,false,false
 dashboardDsAdHocFiltering,experimental,@grafana/datapro,false,false,true
+alertmanagerRemoteSecondaryWithRemoteState,experimental,@grafana/alerting-squad,false,false,false