Alerting: Add configuration options to migrate to an external Alertmanager (#71318)

* add configuration options to .ini file and parse them

* updates on config options, add external AM config to the main config struct

* separate external AM configs from general alerting configs, naming

* comments about usage of tenantID in basic auth & not using config options yet

pkg/setting/setting_unified_alerting.go

@@ -95,10 +95,20 @@ type UnifiedAlertingSettings struct {
 	Screenshots                   UnifiedAlertingScreenshotSettings
 	ReservedLabels                UnifiedAlertingReservedLabelSettings
 	StateHistory                  UnifiedAlertingStateHistorySettings
+	RemoteAlertmanager            RemoteAlertmanagerSettings
 	// MaxStateSaveConcurrency controls the number of goroutines (per rule) that can save alert state in parallel.
 	MaxStateSaveConcurrency int
 }
 
+// RemoteAlertmanagerSettings contains the configuration needed
+// to disable the internal Alertmanager and use an external one instead.
+type RemoteAlertmanagerSettings struct {
+	Enable   bool
+	URL      string
+	TenantID string
+	Password string
+}
+
 type UnifiedAlertingScreenshotSettings struct {
 	Capture                    bool
 	CaptureTimeout             time.Duration
@@ -337,6 +347,15 @@ func (cfg *Cfg) ReadUnifiedAlertingSettings(iniFile *ini.File) error {
 		uaCfg.DefaultRuleEvaluationInterval = uaMinInterval
 	}
 
+	remoteAlertmanager := iniFile.Section("remote.alertmanager")
+	uaCfgRemoteAM := RemoteAlertmanagerSettings{
+		Enable:   remoteAlertmanager.Key("enabled").MustBool(false),
+		URL:      remoteAlertmanager.Key("url").MustString(""),
+		TenantID: remoteAlertmanager.Key("tenant").MustString(""),
+		Password: remoteAlertmanager.Key("password").MustString(""),
+	}
+	uaCfg.RemoteAlertmanager = uaCfgRemoteAM
+
 	screenshots := iniFile.Section("unified_alerting.screenshots")
 	uaCfgScreenshots := uaCfg.Screenshots