Field 支持自定义字段,如：NewRawField(sql string,vars ...interface{})

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Describe the feature

通过如下方法实现自定义字段
var customCol1 = field.NewFieldRaw("IF(column1='1',column1,?)","456")
var customCol2 = field.NewFieldRaw("case column2 when ? then ? else ? end","1","正常","异常")
query.Table1.Select(customCol1,customCol2,query.Table1.Column2).Order(customCol1.Desc()).Where(customCol2.Eq(value))

Motivation

select 时需要自定义字段
order by 时 需要通过自定义字段进行排序
where 时 需要通过自定义字段筛选

Related Issues

#1113
#1028

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#1213