Go codegen struct JSON tags

The json serialization doesn't handle lowercase/underscored column names when serializing to JSON. The struct fields are missing json tags to convert into lowercase. This might not really be a problem when GraphQL camelcase naming conventions are used, but databases might use lowercase/underscores by default.

```go
type AutoGenerated struct {
Name string `json:"name"`
}
```

https://github.com/hasura/codegen-assets/blob/master/builder-kit/src/languages-functional/go.ts#L25