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.go

@@ -91,7 +91,7 @@ func (api *API) RegisterAPIEndpoints(m *metrics.API) {
 	api.RegisterAlertmanagerApiEndpoints(NewForkedAM(
 		api.DatasourceCache,
 		NewLotexAM(proxy, logger),
-		&AlertmanagerSrv{store: api.AlertingStore, mam: api.MultiOrgAlertmanager, secrets: api.SecretsService, log: logger, ac: api.AccessControl},
+		&AlertmanagerSrv{crypto: api.MultiOrgAlertmanager.Crypto, log: logger, ac: api.AccessControl, mam: api.MultiOrgAlertmanager},
 	), m)
 	// Register endpoints for proxying to Prometheus-compatible backends.
 	api.RegisterPrometheusApiEndpoints(NewForkedProm(