[Feature Request] datePicker组件type为daterange的类型加一个footer插槽

### Existing Component
是

### Component Name
datePicker

### Description
在做项目的过程中遇到了这样一个需求，用户选择了一个日期范围，然后开发人员要根据规则自动生成它的对比日期，在用户点击确认之后，这个对比日期才生效，就需要在datePicker组件type为daterange的类型加一个footer插槽

Start at the datePicker component entry point and inspect how the daterange footer is currently rendered and whether other slots provide a pattern. Add a footer slot for the daterange type, then verify that the slot can support the described post-selection comparison-date workflow and that existing datePicker behavior remains unchanged.

Written by the indexing model from the issue text.