enum Default() should support non-string GoType

- [x] I have searched the [issues](https://github.com/ent/ent/issues) of this repository and believe that this is not a duplicate.

## Summary 💡

enum Default() should take an `interface{}` parameter to support non-string GoType enum.

```go
// Default sets the default value of the field.
func (b *enumBuilder) Default(value interface{}) *enumBuilder {
b.desc.Default = value
return b
}
```

## Motivation 🔦

schema definition:
```go

type DeliveryBy uint8

const (
DeliveryByUnknown DeliveryBy = iota
DeliveryByShop
)

// implement the EnumValues and ValueScanner interfaces

func (Shop) Fields() []ent.Field {
return []ent.Field{
field.Enum("delivery_by").GoType(DeliveryBy(0)).Default(DeliveryByShop.String()),
}
}
```

generates a string constant causing compile error:

```go
const DefaultDeliveryBy DeliveryBy = "shop"
```

> ent/shop/shop.go:123:7: cannot use "shop" (type untyped string) as type DeliveryBy

Thanks for your brilliant library!