Now you can use big.Float for numeric type.
```go
type MyMoney struct {
Id int64
Money big.Float `xorm:"numeric(22,2)"`
}
```
Reviewed-on: https://gitea.com/xorm/xorm/pulls/1973
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-committed-by: Lunny Xiao <xiaolunwen@gmail.com>
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