OnClick handler for Picker

[warmhug]

ref https://github.com/ant-design/ant-design-mobile/issues/1841

What problem does this feature solve?

The user can change the value of the Picker/ PickerView/ DatePicker by clicking the value and scroll to that value besides scrolling the picker.

What does the proposed API look like?

<Picker onPickerClick={(value) => console.log(value) />

onPickerClick will return the value that is clicked.

[vipcxj]

就算不给做这个，为PickerItem加个OnClick属性，Picker则提供一个ScrollTo方法也好啊