Add Clear function to Date and Time pickers. (#13082)

* feat: add Clear function to Date and Time pickers.

* feat: update comment wording.

---------

Co-authored-by: rabbitism <dongbin@irihi.tech>

src/Avalonia.Controls/CalendarDatePicker/CalendarDatePicker.cs

@@ -918,5 +918,13 @@ namespace Avalonia.Controls
         {
             return !string.IsNullOrWhiteSpace(formatString);
         }
+
+        /// <summary>
+        /// Clear <see cref="SelectedDate"/>.
+        /// </summary>
+        public void Clear()
+        {
+            SetCurrentValue(SelectedDateProperty, null);
+        }
     }
 }

src/Avalonia.Controls/DateTimePickers/DatePicker.cs

@@ -410,5 +410,13 @@ namespace Avalonia.Controls
         {
             SelectedDateChanged?.Invoke(sender, e);
         }
+
+        /// <summary>
+        /// Clear <see cref="SelectedDate"/>.
+        /// </summary>
+        public void Clear()
+        {
+            SetCurrentValue(SelectedDateProperty, null);
+        }
     }
 }

src/Avalonia.Controls/DateTimePickers/TimePicker.cs

@@ -283,5 +283,13 @@ namespace Avalonia.Controls
             _popup!.Close();
             SetCurrentValue(SelectedTimeProperty, _presenter!.Time);
         }
+
+        /// <summary>
+        /// Clear <see cref="SelectedTime"/>.
+        /// </summary>
+        public void Clear()
+        {
+            SetCurrentValue(SelectedTimeProperty, null);
+        }
     }
 }