Fully specify custom type

Within the Generator there is currently no way to fully override custom types for lists, maps, objects and sets. For instance:

```go
case e.List != nil:
if e.List.CustomType != nil {
return fmt.Sprintf("%s{\nElemType: %s,\n}", e.List.CustomType.Type, GetElementType(e.List.ElementType))
```

Once this has been [addressed in the specification](https://github.com/hashicorp/terraform-plugin-codegen-spec/issues/21), then the code handling custom types for list, map, object and set will need updating in the generator.