feat(alerting): began work on splitting scheduler into scheduler and engine

pkg/services/alerting/interfaces.go

@@ -0,0 +1,12 @@
+package alerting
+
+import "time"
+
+type Executor interface {
+	Execute(rule *AlertJob, resultChan chan *AlertResult)
+}
+
+type Scheduler interface {
+	Tick(time time.Time, execQueue chan *AlertJob)
+	Update(rules []*AlertRule)
+}