Feat: [date-pick] support footer slot

[wuhaohuang]

Please make sure these boxes are checked before submitting your PR, thank you!

• Make sure you follow contributing guide English | (中文 | Español | Français).
• Make sure you are merging your commits to dev branch.
• Add some descriptions and refer to relative issues for your PR.

This feat is to support date-pick’s ability to customize the footer, allowing developers to completely customize the bottom area of the calendar through slots, similar to antd-vue’s date selection component.
这个功能是为了支持 date-pick 能自定义 footer，允许开发人员能通过 slot 的方式完全自定义日历最下方的区域，类似 antd-vue 的日期选择组件。

Two test cases were also written to see whether it meets expectations with or without footer slot.
编写了两个测试用例，关于有无使用 footer slot 的情况下是否符合期望。

how to use:

like:

thx~

[pull-request-triage]

👋 @wuhaohuang, seems like this is your first time contribution to element-plus.

• Please make sure that you have read our guidelines and code of conduct before making a contribution.
• You can comment with /label Components:[component_name] to add a label for which component you are working on.
• You may join our for staying tuned.

[github-actions]

[github-actions]

Hello @wuhaohuang, thank you for contributing to element-plus, please see our guideline to see how to make contribution