feat(core): run the cache in a dedicated handler function

Add a standalone cache handler function that owns the incremental cache, the
tag cache and CDN invalidation, and a `cache` override that other functions use
to reach it.

- `adapters/cache-adapter.ts` exposes `GET/PUT/DELETE /cache/*` and
  `POST /cache/revalidate-tags` through `createGenericHandler`.
- `build/createCacheBundle.ts` bundles it to `.open-next/cache-function`, wired
  into `buildAdapter` behind the new `skipCache` option and reported as
  `cacheFunction` in the OpenNext output.
- New top level `cacheHandler` config option carries the incremental cache, tag
  cache and cdn invalidation overrides for that function.
- New `cache` override with `fetch`, `local` and `dummy` implementations, and a
  shared wire format in `utils/cache-get.ts` that splits the entry metadata into
  headers and the payload into the body.

Nothing is removed here: the existing in-process `incrementalCache` and
`tagCache` overrides keep working unchanged.

Author: conico974

.changeset/cache-handler-function.md

@@ -0,0 +1,31 @@
+---
+"@opennextjs/core": minor
+---
+
+Add a dedicated cache handler function
+
+The incremental cache, the tag cache and CDN invalidation can now run in their own function, bundled
+to `.open-next/cache-function` and reachable over the new `cache` override.
+
+The overrides for that function are configured through a new top level `cacheHandler` option:
+
+```ts
+export default {
+	default: {
+		override: {
+			cache: "local",
+		},
+	},
+	cacheHandler: {
+		incrementalCache: "s3",
+		tagCache: "dynamodb",
+	},
+} satisfies OpenNextConfig;
+```
+
+Three `cache` implementations ship with core: `fetch` (HTTP), `local` (in-process, imports the cache
+bundle) and `dummy`. Adapters that do not want the bundle can opt out with `skipCache` in
+`buildAdapter`.
+
+This is additive: the existing `default.override.incrementalCache` and `default.override.tagCache`
+keep working unchanged.