[ACTIONS] on.schedule: create a new payload

do not reuse the payload of the event that triggered the creation of
the scheduled event. Create a new one instead that contains no other
information than the event name in the action field ("schedule").

(cherry picked from commit 0b40ca1ea5)
(cherry picked from commit f86487432b)
(cherry picked from commit 4bd5d2e9d0)

modules/structs/hook.go

@@ -402,6 +402,16 @@ func (p *PullRequestPayload) JSONPayload() ([]byte, error) {
 	return json.MarshalIndent(p, "", "  ")
 }
 
+type HookScheduleAction string
+
+const (
+	HookScheduleCreated HookScheduleAction = "schedule"
+)
+
+type SchedulePayload struct {
+	Action HookScheduleAction `json:"action"`
+}
+
 // ReviewPayload FIXME
 type ReviewPayload struct {
 	Type    string `json:"type"`