[Feature Request] Request scoped slot in el-select for custom rendering of selected items as a child of $ref="tags"

### Existing Component
Yes

### Component Name
el-select

### Description
Usecase 1: in country multiselect, country flag has to be renderend along with selected country text
Usecase 2: in tag multiselect, tag color selected by user while created has to be renderend along with selected tag name

If a scoped slot is provided as a child of $ref="tags", we can customize the rendering portion of the selected area