Change ScheduleSpec to Datetime

[FireDefend]

Hi team, do you think it is possible to use datetime type instead of ScheduleSpec? Arm team is suggesting us to change it.

type ScheduleSpec struct {
Date string json:"date"
Time string json:"time"
Zone string json:"zone"
}

type StageSpec struct {
...
Schedule *ScheduleSpec json:"schedule,omitempty"
}

---

type StageSpec struct {
...
Schedule time.Time json:"datetime"
}

[Haishi2016]

Think of usability scenarios: Schedule a deployment at 5PM on Friday (local time).

[FireDefend]

OpenAPI spec is using ISO 8601. It will support offset date time(local time). I think we can use the same standard to parse in symphony code. https://www.iso.org/iso-8601-date-and-time-format.html

[FireDefend]

fixed by the PR: #276