Customizable end-task prompts

[mmertsock]

Adds an optional ORK1TaskViewControllerDelegate method to customize the "End Task"/"Discard Results" prompts shown when exiting a survey. In support of https://github.com/CareEvolution/MyDataHelps-iOS/pull/1399

Note that this was only needed for ORK1Kit, not ResearchKit v2, since these are used by the special TaskManager workflows that only use ORK1Kit. Actual ResearchKit surveys, for example, use other task delegates (e.g. RK1SurveyTaskInteraction) that already have their own taskViewControllerShouldConfirmCancel logic, and we don't currently have any requests to customize the button text for their end-task prompts.

Testing

This is a dependency for https://github.com/CareEvolution/MyDataHelps-iOS/pull/1399 and all testing would be performed as part of that PR.

[eschramm]

LGTM 👍

[mmertsock]

Refactored this with a cleaner design: declared a @protocol with (all optional) properties for the various customizable button labels. This allows us to have just a single new (also optional) ORK1TaskViewControllerDelegate method that returns an object conforming to that protocol. Ready for re-review.

[eschramm]

Changes look nice - I like this approach 🚢