`FormValueControl` and `FormCheckboxControl` no longer require that
`value` and `checked` are implemented as `model()` signals. The
interfaces have been relaxed to accept separate `input()` and `output()`
definitions to facilitate integration with existing controls.

When designing a new control, `model()` should be preferred.

```ts
// Preferred
class implements FormValueControl<string> {
  readonly value: model.required<string>();
}

// Okay
class MyFormControl implements FormValueControl<string> {
  readonly value: input.required<string>();
  readonly valueChange: output<string>();
}
```

Fix angular#65478