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/schedule/schedule_unit_test.go

@@ -1015,7 +1015,7 @@ func setupScheduler(t *testing.T, rs store.RuleStore, is store.InstanceStore, ac
 	m := metrics.NewNGAlert(registry)
 	secretsService := secretsManager.SetupTestService(t, fakes.NewFakeSecretsStore())
 	decryptFn := secretsService.GetDecryptedValue
-	moa, err := notifier.NewMultiOrgAlertmanager(&setting.Cfg{}, &notifier.FakeConfigStore{}, &notifier.FakeOrgStore{}, &notifier.FakeKVStore{}, decryptFn, m.GetMultiOrgAlertmanagerMetrics(), nil, log.New("testlogger"))
+	moa, err := notifier.NewMultiOrgAlertmanager(&setting.Cfg{}, &notifier.FakeConfigStore{}, &notifier.FakeOrgStore{}, &notifier.FakeKVStore{}, decryptFn, m.GetMultiOrgAlertmanagerMetrics(), nil, log.New("testlogger"), secretsService)
 	require.NoError(t, err)
 
 	schedCfg := SchedulerCfg{