[NATS transport] Support for headers in EncodeJSONResponse

### What would you like?

# Overview
Since release `v1.11.0`, nats.go supports message headers. headers are very useful to transfer metadata together with messages.

I'm proposing to add basic support for headers in the default response encoder.

# Proposed API

The idea is to do something similar it's done for `transport/http` with the `Headerer` interface. API change proposal is backward compatible (no public API change)

```go
type Headerer interface {
Headers() Header
}

func EncodeJSONResponse(_ context.Context, reply string, nc *nats.Conn, response interface{}) error {
msg := nats.NewMessage(reply)
if nc. HeadersSupported() {
if headerer, ok := response.(Headerer); ok {
msg.Header = headerer.Headers() // I wrote that to make it shorter
}
}

msg.Data, err := json.Marshal(response)
if err != nil {
return err
}

return nc.PublishMsg(msg)
}
```

If the proposal is accepted I can raise a PR