Refactor the request schedule block

[Zarquan]

Current data model for a schedule block has preparing, executing and finishing in the request.
User wants to specify a single duration, an array of start times for executing, and then leave the rest to the service to make offers.

[Zarquan]

OpenAPI request model has been updated to skip preparing and finishing, and include a list for executing.

name: Test request
executable:
  name: Test executable
  type: urn:jupyter-notebook-0.1
  notebook: https://raw.githubusercontent.com/Zarquan/binder-demo/main/02_dmhh.ipynb
schedule:
  requested:
      start:
      - 2024-10-12T02:34:51+00:00/PT0S
      - 2024-10-12T02:34:51+00:00/PT0H
      - 2024-10-12T02:34:51+00:00/PT1H
      - 2024-10-13T02:34:51+00:00/PT1H
      - 2024-10-14T02:34:51+00:00/PT1H
      duration: PT1H

[Zarquan]

Current parser requires something in the range part of the start interval.
https://stackoverflow.com/questions/29823129/what-is-the-canonical-representation-of-zero-in-iso-8601-durations

For a date/time with no duration, we still have to include something in the duration part:

- 2024-10-12T02:34:51+00:00/PT0S

So none of these are valid:

- 2024-10-12T02:34:51+00:00
- 2024-10-12T02:34:51+00:00/
- 2024-10-12T02:34:51+00:00/P
- 2024-10-12T02:34:51+00:00/P0
- 2024-10-12T02:34:51+00:00/PT
- 2024-10-12T02:34:51+00:00/PT0

Do we allow a 'bare' date/time without a range, and fill in the /PT0S part ourselves?
So this:

- 2024-10-12T02:34:51+00:00

is expanded to:

- 2024-10-12T02:34:51+00:00/PT0S