add ETag and Last-Modified to the handler.ts

In the handler.ts you are adding following headers:
```
headers: {
      'Content-Type': ContentType,
      'Content-Disposition': ContentDisposition,
      'Cache-Control': 'max-age=31536000',
    },
```
It would be great if you could add ETag and Last-Modified as well to improve the caching options and increase a better SEO rating.
Thanks!