Is is possible to get field directives in mutation hook?

### What happened?

According to https://gqlgen.com/recipes/modelgen-hook/ we can mutate generated models file.

### What did you expect?
I want to add some tags into struct fields depending on existing directives.

### Minimal graphql.schema and models to reproduce

```gql
directive @testDirective(value: Int!) on INPUT_FIELD_DEFINITION

input SomeInput {
someField: Int! @testDirective(value: 100)
}
```

Expected struct:

```go
type SomeInput struct {
SomeField int `json:"someField" test:"100"`
}
```

### versions
- `gqlgen version`? 0.12.2
- `go version`? 1.15
- dep or go modules? go.mod