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/Makefile

@@ -0,0 +1,9 @@
+include ../../sdk.mk
+
+.PHONY: generate # Run Grafana App SDK code generation
+generate: install-app-sdk update-app-sdk
+	@$(APP_SDK_BIN) generate \
+		--source=./kinds/ \
+		--gogenpath=./pkg/apis \
+		--grouping=group \
+		--defencoding=none