This solves the main use case from Issue #19914.
Previously, we would set the `Cache-Control` header to a constant and rely on the server cache. This would mean the browser would always request the image and the server could response with 304 Not Modified to omit the response body.
This PR changes the behavior such that the `max-age` will propagate from the upstream server to the Next.js Image Optimization Server and allow browser caching. ("upstream" meaning external server or just an internal route to an image)
This PR does not change the `max-age` for static imports which will remain `public, max-age=315360000, immutable`.
#### Pros:
- Fewer HTTP requests after initial browser visit
- User configurable `max-age` via the upstream image `Cache-Control` header
#### Cons:
- ~~Might be annoying for `next dev` when modifying a source image~~ (solved: use `max-age=0` for dev)
- Might cause browser to cache longer than expected (up to 2x longer than the server cache if requested in the last second before expiration)
## Bug
- [x] Related issues linked using `fixes #number`
