[feature request] it is recommended that the El select multi select component add the full select function

### Existing Component
是

### Component Name
el-select

### Description
建议el-select多选组件增加全选功能，有全选选项, 在很多需求场景下，如果选项很多，需要一个一个去勾选，操作有些不方便。
（It is recommended that the el-select multi-select component add the full-select function. There are full-select options. In many demand scenarios, if there are many options, you need to check them one by one, which is a bit inconvenient.）