Cache: run the cache in a dedicated handler function

.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.

packages/core/src/adapters/cache-adapter.ts

@@ -0,0 +1,8 @@
+import { createGenericHandler } from "../core/createGenericHandler.js";
+
+import { handler as cacheHandler } from "./cache-handler.js";
+
+export const handler = await createGenericHandler({
+	handler: cacheHandler,
+	type: "cache",
+});