[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/test-data/post-user-config.json

@@ -8,11 +8,13 @@
             "resolve_timeout": "1s",
             "smtp_smarthost": ""
         },
-        "route": {},
+        "route": {
+            "receiver": "grafana_managed"
+        },
         "templates": [],
         "receivers": [
             {
-                "name": "",
+                "name": "grafana_managed",
                 "grafana_managed_receiver_configs": [
                     {
                         "uid": "alertmanager UID",