[IMP] web: Expose datepicker configuration to view declarations

The lower-level widget itself supports passing options to the underlying
datepicker library. However this is not exposed to higher-level view
declaration. With this patch, one can declare a field in a view such as:

```xml
<field name="datefield" options='{"daysOfWeekDisabled": [0, 6]}'/>
```

That option would land in the widget and disable those weekdays.

Documentation has been updated accordingly.

Closes #25044.

doc/reference/javascript_reference.rst

@@ -1329,11 +1329,27 @@ order.
 
     - Supported field types: *date*, *datetime*
 
+    Options:
+
+    - datepicker: extra settings for the datepicker_ widget.
+
+    .. code-block:: xml
+
+        <field name="datefield" options='{"datepicker": {"daysOfWeekDisabled": [0, 6]}}'/>
+
 - datetime (FieldDateTime)
     This is the default field type for fields of type *datetime*.
 
     - Supported field types: *date*, *datetime*
 
+    Options:
+
+    - datepicker: extra settings for the datepicker_ widget.
+
+    .. code-block:: xml
+
+        <field name="datefield" options='{"datepicker": {"daysOfWeekDisabled": [0, 6]}}'/>
+
 - monetary (FieldMonetary)
     This is the default field type for fields of type 'monetary'. It is used to
     display a currency.  If there is a currency fields given in option, it will
@@ -1814,4 +1830,6 @@ Relational fields
 .. _event delegation:
     http://api.jquery.com/delegate/
 
+.. _datepicker: https://github.com/Eonasdan/bootstrap-datetimepicker
+
 .. _deferred: http://api.jquery.com/category/deferred-object/