Alerting: Refactor GET/POST alerting config routes to be more extensible (#47229)

* Refactor GET am config to be extensible

* Extract post config route

* Fix tests

* Remove temporary duplication

* Fix broken test due to layer shift

* Fix duplicated error message

* Properly return 400 on config rejection

* Revert weird half method extraction

* Move things to notifier package and avoid redundant interface

* Simplify documentation

* Split encryption service and depend on minimal abstractions

* Properly initialize things all the way up to the composition root

* Encryption -> Crypto

* Address misc feedback

* Missing docstring

* Few more simple polish improvements

* Unify on MultiOrgAlertmanager. Discover bug in existing test

* Fix rebase conflicts

* Misc feedback, renames, docs

* Access crypto hanging off MultiOrgAlertmanager rather than having a separate API to initialize

pkg/services/ngalert/api/api_alertmanager_test.go

@@ -357,7 +357,8 @@ func createSut(t *testing.T, accessControl accesscontrol.AccessControl) Alertman
 		accessControl = acMock.New().WithDisabled()
 	}
 	log := log.NewNopLogger()
-	return AlertmanagerSrv{mam: mam, store: &configStore, secrets: secrets, ac: accessControl, log: log}
+	crypto := notifier.NewCrypto(secrets, &configStore, log)
+	return AlertmanagerSrv{mam: mam, crypto: crypto, ac: accessControl, log: log}
 }
 
 func createAmConfigRequest(t *testing.T) apimodels.PostableUserConfig {
@@ -395,7 +396,7 @@ func createMultiOrgAlertmanager(t *testing.T) *notifier.MultiOrgAlertmanager {
 		}, // do not poll in tests.
 	}
 
-	mam, err := notifier.NewMultiOrgAlertmanager(cfg, &configStore, &orgStore, kvStore, decryptFn, m.GetMultiOrgAlertmanagerMetrics(), nil, log.New("testlogger"))
+	mam, err := notifier.NewMultiOrgAlertmanager(cfg, &configStore, &orgStore, kvStore, decryptFn, m.GetMultiOrgAlertmanagerMetrics(), nil, log.New("testlogger"), secretsService)
 	require.NoError(t, err)
 	err = mam.LoadAndSyncAlertmanagersForOrgs(context.Background())
 	require.NoError(t, err)