Use sql.NullString & Co. declaring them in gqlgen.yml config file

### What happened?

I'm trying to avoid pointers as my structs fields.

But `gqlgen` generates struct like:

```go
type User struct {
ID uint64 `json:"id"`
Nickname string `json:"lastname"`
Img *string `json:"img"`
}
```

with `*string` as you can see.

### What did you expect?

I would like to use something like this in `gqlgen.yml` config file:

```yml
models:
String:
model:
- github.com/99designs/gqlgen/graphql.String
- github.com/99designs/gqlgen/graphql.NullString
# or maybe also:
- database/sql.NullString
```

So to have this instead:

```go
type User struct {
ID uint64 `json:"id"`
Nickname string `json:"lastname"`
Img sql.NullString `json:"img"`
}
```