Let the options of a select change while the form is open

[KamilSztandur]

Problem

The list of options in a select is fixed when the field is created. There is no way to swap it later. That rules out the ordinary dependent dropdown: the instructors you can pick depend on the aircraft you chose, the cities depend on the country, the list arrives from the server after the form is already on screen. Someone porting two real forms had to give up on the built-in select for this. It is a gap, not a design choice.

Solution

setOptions(newList) on both select controllers. It replaces the list and takes care of the value:

• A selected value that is no longer on the list is cleared (on the multi select, dropped from the set). This counts as the program's write, like prefill, not as a user edit. So a prefilled field the user never touched stays quiet until submit, while a field the user had picked from shows its "required" error right away.
• A read-only field is cleared too, because a value that is not on the list cannot be shown in the dropdown.
• Widgets rebuild even when the value stayed the same, so the new list appears.

Typical use is a listener on the other field:

aircraft.addListener(() {
  instructor.setOptions(instructorsFor(aircraft.fieldValue));
});

One side effect: the single select now copies the list it is given, which the multi select already did. options is read-only on both.

In this PR

• setOptions on AdvancedSingleSelectFieldController and AdvancedMultiSelectFieldController.
• Tests: list replaced and listeners notified with the value unchanged, off-list value cleared or dropped, no error on an untouched field, error at once on an edited one, read-only cleared, the list is a copy, disposed throws.
• Docs: the single and multi select sections, the select rendering page, a FAQ entry with the == warning about server copies of options, SKILL.md and a changelog entry.

flutter analyze clean, all 271 tests pass.

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