next/image: support custom cache handlers (#88248)
When self-hosting Next.js, it's possible to customize a cache handler
for ISR. Self-hosted Next.js also makes use of a similar cache interface
for `next/image`, but there's no way to customize this cache handler.
This plugs into the [existing
ability](https://nextjs.org/docs/app/api-reference/config/next-config-js/incrementalCacheHandlerPath)
to customize a `cacheHandler` to support the `IMAGE` type. This lets you
implement whatever constraints you might have on your self-hosted
environment, eg the need for an LRU cache, or storing optimized images
outside of disk.
This is currently opt-in because it's a breaking change for cache
handlers to start receiving `IMAGE` cache types, which we'll make
default in the next major.
