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/checkpointer.go

@@ -0,0 +1,30 @@
+package hookdeliveryforwarder
+
+import "time"
+
+type Checkpointer interface {
+	GetOrCreate(hookID int64) (*State, error)
+	Update(hookID int64, pos *State) error
+}
+
+type InMemoryCheckpointer struct {
+	t  time.Time
+	id int64
+}
+
+func (p *InMemoryCheckpointer) GetOrCreate(hookID int64) (*State, error) {
+	return &State{DeliveredAt: p.t}, nil
+}
+
+func (p *InMemoryCheckpointer) Update(hookID int64, pos *State) error {
+	p.t = pos.DeliveredAt
+	p.id = pos.ID
+
+	return nil
+}
+
+func NewInMemoryLogPositionProvider() Checkpointer {
+	return &InMemoryCheckpointer{
+		t: time.Now(),
+	}
+}