[Alerting]: Alertmanager API implementation (#32174)

* Add validation for grafana recipient

* Alertmanager API implementation (WIP)

* Fix encoding/decoding receiver settings from/to YAML

* Save templates together with the configuration

* update POST to apply latest config

* Alertmanager service enabled by the ngalert toggle

* Silence API integration with Alertmanager

* Apply suggestions from code review

Co-authored-by: gotjosh <josue@grafana.com>
Co-authored-by: Ganesh Vernekar <15064823+codesome@users.noreply.github.com>

pkg/services/ngalert/api/util.go

@@ -35,10 +35,6 @@ func stringPtr(s string) *string {
 	return &s
 }
 
-func boolPtr(b bool) *bool {
-	return &b
-}
-
 func backendType(ctx *models.ReqContext, cache datasources.CacheService) (apimodels.Backend, error) {
 	recipient := ctx.Params("Recipient")
 	if recipient == apimodels.GrafanaBackend.String() {