[Feature Request] el-select Can add a required check for options？ 下拉框选项能否增加必填required属性的校验

### Existing Component
是

### Component Name
el-select

### Description
As the title，Sometimes I need to check the drop-down box, but the required attribute does not seem to support the select tag. Is there any compatible solution?
如题，下拉框我们需要必填，但required好像不支持select标签。element能增加下必填属性的兼容方案下嘛