Making `myOptionalProp` Optional Without S.extend and S.partial

const MyType = S.Struct({
  myPropA: S.String,
  myPropB: S.String,
  myPropC: S.String,
  myOptionalProp: S.propertySignature(S.String).pipe(
    S.fromKey('my-optional-prop'),
  ),
});