✨ [Feature]: DatePicker新增footer插槽

[AcWrong02]

What problem does this feature solve

我在使用日历组件时，需要直接在日历面板的下方添加其他元素，但是当前的datePicker组件是不支持的，只能通过控制css样式来实现，那对开发者来说太痛苦了

What does the proposed API look like

<datePicker>
    <template #footer>
      // 这里添加自定义的fooer内容
    </template>
</datePicker>

What is your project name

null

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Title: ✨ [Feature]: DatePicker adds footer slot