Ensure that the old UI (to display schedules) works with REST APIs

Design:

• All add/update APIs for schedules already are backward-compatible with old ScheduleSpecification/ScheduleUpdateDetail

• get/list schedule APIs now return ScheduleDetail instead of ScheduleSpecification. To allow retrieving the latter, we will add an optional URL parameter ?format=spec to these APIs.

• the UI will have to be changed to add that parameter to its schedule requests.

PR: https://github.com/caskdata/cdap/pull/8924