Breaking change: Add ScanSchedule, allows interval and cron based configurations.

See https://pkg.go.dev/github.com/robfig/cron#hdr-CRON_Expression_Format for expression syntax.

`ScanInterval` will still work for the time being. The only situation it does not work is when you want to disable periodic scanning by setting `ScanInterval=0`. If you want to disable it, please set `ScanSchedule=""`

Closes #1085

scheduler/scheduler.go

@@ -0,0 +1,34 @@
+package scheduler
+
+import (
+	"context"
+
+	"github.com/robfig/cron/v3"
+)
+
+type Scheduler interface {
+	Run(ctx context.Context)
+	Add(crontab string, cmd func()) error
+}
+
+func New() Scheduler {
+	c := cron.New(cron.WithLogger(&logger{}))
+	return &scheduler{
+		c: c,
+	}
+}
+
+type scheduler struct {
+	c *cron.Cron
+}
+
+func (s *scheduler) Run(ctx context.Context) {
+	s.c.Start()
+	<-ctx.Done()
+	s.c.Stop()
+}
+
+func (s *scheduler) Add(crontab string, cmd func()) error {
+	_, err := s.c.AddFunc(crontab, cmd)
+	return err
+}