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/app.go

@@ -0,0 +1,45 @@
+package app
+
+import (
+	"github.com/grafana/grafana-app-sdk/app"
+	"github.com/grafana/grafana-app-sdk/simple"
+
+	"github.com/grafana/grafana/apps/alerting/historian/pkg/apis/alertinghistorian/v0alpha1"
+	"github.com/grafana/grafana/apps/alerting/historian/pkg/app/config"
+)
+
+func New(cfg app.Config) (app.App, error) {
+	runtimeConfig := cfg.SpecificConfig.(config.RuntimeConfig)
+
+	simpleConfig := simple.AppConfig{
+		Name:       "alerting.historian",
+		KubeConfig: cfg.KubeConfig,
+		VersionedCustomRoutes: map[string]simple.AppVersionRouteHandlers{
+			"v0alpha1": {
+				{
+					Namespaced: true,
+					Path:       "/alertstate/history",
+					Method:     "GET",
+				}: runtimeConfig.GetAlertStateHistoryHandler,
+			},
+		},
+		// TODO: Remove when SDK is fixed.
+		ManagedKinds: []simple.AppManagedKind{
+			{
+				Kind: v0alpha1.DummyKind(),
+			},
+		},
+	}
+
+	a, err := simple.NewApp(simpleConfig)
+	if err != nil {
+		return nil, err
+	}
+
+	err = a.ValidateManifest(cfg.ManifestData)
+	if err != nil {
+		return nil, err
+	}
+
+	return a, nil
+}