title: flutter disable certain days of the week when showing date picker based on a list
body: <p>I have a list showing days of the week. 1 is for Monday, 2 for Tuesday, 3 for Wednesday etc.</p>
  <pre><code>List&lt;int&gt;? daysOfWeek = [1,2,3];
  </code></pre>
  <p>When I use the showDatePicker how do I use selectableDayPredicate: to disable those days of the week.</p>
  <pre><code>                return GestureDetector(
                    onTap: () =&gt; showDatePicker(
                      context: context,
                      initialDate: DateTime.now(),
                      firstDate: DateTime(2021),
                      lastDate: DateTime(2023),
                      selectableDayPredicate: ????
                    ),
  </code></pre>
  <p>Thanks</p>