Add back optional record fields for 4 exercises

[ageron]

This PR makes some fields optional for four exercises that used to have optional fields: clock, killer-sudoku-helper, rest-api and robot-simulator.

This makes the tests nicer to read, and gives the user some experience with default fields.

For clock, killer-sudoku-helper and robot-simulator, the optional field has a default value, which is preconfigured in the exercise stub, so this doesn't make the exercise any harder. In the case of rest-api, the payload field for the get function is optional without any default value, so the user needs to handle the Err(MissingField) case.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!