Add POC of GitHub Webhook Delivery Forwarder (#682)

* Add POC of GitHub Webhook Delivery Forwarder

* multi-forwarder and ctrl-c existing and fix for non-woring http post

* Rename source files

* Extract signal handling into a dedicated source file

* Faster ctrl-c handling

* Enable automatic creation of repo hook on startup

* Add support for forwarding org hook deliveries

* Set hook secret on hook creation via envvar (HOOK_SECRET)

* Fix org hook support

* Fix HOOK_SECRET for consistency

* Refactor to prepare for custom log position provider

* Refactor to extract inmemory log position provider

* Add configmap-based log position provider

* Rename githubwebhookdeliveryforwarder to hookdeliveryforwarder

* Refactor to rename LogPositionProvider to Checkpointer and extract ConfigMap checkpointer into a dedicated pkg

* Refactor to extract logger initialization

* Add hookdeliveryforwarder README and bump go-github to unreleased ver

pkg/hookdeliveryforwarder/logger.go

@@ -0,0 +1,17 @@
+package hookdeliveryforwarder
+
+import (
+	"fmt"
+	"os"
+)
+
+type logger struct {
+}
+
+func (f logger) Logf(format string, args ...interface{}) {
+	fmt.Fprintf(os.Stdout, format+"\n", args...)
+}
+
+func (f logger) Errorf(format string, args ...interface{}) {
+	fmt.Fprintf(os.Stderr, format+"\n", args...)
+}