Alerting: Foundations of historian app. (#114463)

We have two historians in alerting - alert state and notification. The intention
of this app is to provide query capabilities for both.

In this initial commit, the existing /history API is simply cloned to the new
app. It is identical except that it will send Kubernetes-style error responses
instead of Grafana-style.

This approach was taken to implement the new app more iteratively - ideally we
would define a new API, but this requires quite a significant overhaul of the
backend code.

apps/alerting/historian/pkg/app/config/config.go

@@ -0,0 +1,9 @@
+package config
+
+import (
+	"github.com/grafana/grafana-app-sdk/simple"
+)
+
+type RuntimeConfig struct {
+	GetAlertStateHistoryHandler simple.AppCustomRouteHandler
+}