bug: support for overriding id field

[philwinder]

Hi again,
According to the ent docs, you can override the id field to be something other than an int.

If you use a Field with just an id field, it fails to compile:

// Fields of the Model.
func (Model) Fields() []ent.Field {
	return []ent.Field{
		field.String("id"),
	}
}
...
❯ go generate ./...
❯ go run -mod=mod main.go
# todo/ent/ogent
ent/ogent/ogent.go:31:61: undefined: CreateModelReq
ent/ogent/ogent.go:93:61: undefined: UpdateModelReq

If you add some other fields, it compiles, but POST fails:

func (Model) Fields() []ent.Field {
	return []ent.Field{
		field.String("id"),
		field.Time("created_at").
			Optional().
			Default(time.Now),
		field.String("name").
			Optional(),
	}
}
...
❯ curl -X POST http://localhost:8080/models  -H "Content-Type: application/json" -d '{"id": "fdsa90fds-890fgs", "name": "test"}'
{"error_message":"NOT NULL constraint failed: models.id"}% 

If I'm doing something wrong I apologise. If not, it would be great to get support for overriding IDs.

Any workaround in the meantime?