Add formDatePickerStyle property to XLFormDateCell

[nickreffitt]

This PR adds support for changing the datePicker's preferredDatePickerStyle property, and issue raised in #1074

This can be used like so:

row = [XLFormRowDescriptor formRowDescriptorWithTag:kRowDescriptorTagReminderTime
                                            rowType:XLFormRowDescriptorTypeTimeInline
                                              title:@"Remind me at"];
row.cellConfigAtConfigure[@"formDatePickerStyle"] = @(XLFormDateDatePickerStyleWheels);

[mattbold]

Hi @mats-claassen @nickreffitt do you know if a release including this DatePickerStyle functionality is due to be available via cocoapods? I'm keen to make use of it if possible!

[pnkjdubey0]

This fix isn't working anymore. formDatePickerStyle attribute doesn't seem to have any impact anymore.

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