Support for Custom Omittable Type

### What happened?

I saw that #2585 addresses the problem with `undefined` vs `null` in input types, and it's already a great solution for smaller APIs that need to check if a field was explicitly set as `nil` or not defined at all.

However, in my project we have a lot of DTOs and use `github.com/moznion/go-optional.Option` to represent our optional types internally. This allows us to decouple our service methods from the GQL layer.

For smaller projects it would be fine to manually map `graphql.Omittable` to `Option` with a generic function like this:

```go
func MapToOption[T any](o graphql.Omittable[T]) optional.Option[T] {
if o.IsSet() {
return optional.Some(o.Value())
}
return optional.None[T]()
}
```

However, for larger projects it would be nice to use the Modelgen hooks to replace all the `Omittable` with `Option` and a custom unmarshaler/marshaler for that type, but as GQLGen doesn't support this because `nil` values never hit the unmarshaler, I was wondering if an extension to #2585 would be possible with a custom "Omittable" type, in this case `Option`?

The simplest way to allow this would be via a mapper, that users can provide in the config for each `Omittable` type that we want to convert:

```yml
omittableTypes:
*string: mypkg.UnmarshalStringOption
```

### What did you expect?

My proposal above or some other way to convert `Omittable` to `Option`. Entirely replacing `Omittable` with `Option` would be the most elegant.

### Minimal graphql.schema and models to reproduce

```gql
input UpdateTodoInput {
text: String @goField(omittable: true, omittableType: "github.com/moznion/go-optional.Option") # alternative to providing omittable types in the config
}
```

### versions
- `go run github.com/99designs/gqlgen version`?
- `go version`?