Alerting: Add database table for persisting alerting configuration (#32042)

* Alerting: Add database table for persisting alerting configuration

* Fix the linter

* Address review comments

* Don't split templates and configuration

It is already bundled together as part of a of the API so might as well
marshall it directly.

pkg/services/ngalert/models/alertmanager.go

@@ -0,0 +1,20 @@
+package models
+
+import "time"
+
+// AlertConfiguration represents a single version of the Alerting Engine Configuration.
+type AlertConfiguration struct {
+	ID int64 `xorm:"pk autoincr 'id'"`
+
+	AlertmanagerConfiguration string
+	ConfigurationVersion      string
+	CreatedAt                 time.Time `xorm:"created"`
+	UpdatedAt                 time.Time `xorm:"updated"`
+}
+
+// GetLatestAlertmanagerConfigurationQuery is the query to get the latest alertmanager configuration.
+type GetLatestAlertmanagerConfigurationQuery struct {
+	ID int64
+
+	Result *AlertConfiguration
+}