Feature Request: first request WebP, second request: AVIF

# Background

Lamada takes long time to transform image to avif format.

# Solution: Stale-While-Revalidate

[stale-while-revalidate](https://web.dev/articles/stale-while-revalidate) is a caching setting. It’s a parameter in the Cache-Control header, and it tells CDNs how long they can keep serving the image after it expires:

```
Cache-Control: max-age=3600, stale-while-revalidate=60
↑ how long a file can be cached for
↑ how long a CDN can keep serving
the file after max-age expires
```
Here’s how we used it to make sure AVIF never makes image responses slow:

1. First Request: WebP
On the first request, we serve the image as WebP, not as AVIF.

We also set the Cache-Control header to max-age=0, stale-while-revalidate=31536000

2. Immediate Expiry
Because max-age is set to 0, the WebP image expires immediately. This prompts the CDN to forward the second request to us.

3. Second Request: AVIF
When the second request arrives, we serve the image as AVIF.

Responding to the second request can take several seconds because AVIF conversion is slow. But thanks to stale-while-revalidate, our CDN (CloudFront) keeps serving the WebP image until the conversion is complete.

We recognize the second request from the first by the If-None-Match header. Only the second request has it.

When the AVIF image is ready, we return it with Cache-Control: max-age=31536000. This allows the CDN to cache and serve it for a long time.

[# See More](https://www.framer.com/blog/introducing-avif-support/)