[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/components/simplejson/simplejson.go

@@ -482,3 +482,18 @@ func (j *Json) MustUint64(args ...uint64) uint64 {
 
 	return def
 }
+
+// MarshalYAML implements yaml.Marshaller.
+func (j *Json) MarshalYAML() (interface{}, error) {
+	return j.data, nil
+}
+
+// UnmarshalYAML implements yaml.Unmarshaller.
+func (j *Json) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	var data interface{}
+	if err := unmarshal(&data); err != nil {
+		return err
+	}
+	j.data = data
+	return nil
+}