Supporting customize Timer to controll the scheduling behavior

[cloudfly]

Add a new interface Timer replace time.Timer for scheduling the job.

So that the user can create custom timer instead of using standard time.Timer，mainly used for testing。

eg.

import (
	"github.com/benbjohnson/clock"
        "github.com/robfig/cron/v3"
)

func main() {
    mockClock := clock.NewMock()
    cronEngine = cron.New(cron.WithTimerFunc(NewMockTimerFunc(mockClock)))
}

func NewMockTimerFunc(mock *clock.Mock) func(time.Duration) cron.Timer {
	return func(d time.Duration) cron.Timer {
		return &mockTimer{
			timer: mock.Timer(d),
		}
	}
}

type mockTimer struct {
	timer *clock.Timer
}

func (t *mockTimer) C() <-chan time.Time {
	return t.timer.C
}

func (t *mockTimer) Stop() bool {
	return t.timer.Stop()
}