Add client methods SubtasksForListID and SubtasksForTaskID.

[#85033770]
Add client methods CompletedSubtasksForListID and CompletedSubtasksForTaskID.

[#85033772]
Add CompletedAt and CompletedByID fields to task and subtask.
Improve integration test taskContains and listContains methods.

- Only check for ID equality rather than full struct equality.
Add client method Subtask.

[#85033774]