feat(nextjs): Instrument cache handler

[timfish]

Closes #12888

The nextjs custom cache handler is loaded via dynamic import, therefore to wrap its exported class we use import-in-the-middle which is already a dependency of @sentry/node.

Since nextjs is CJS, you can use a sub-module path to instrument the default handler by adding it as a custom cache handler like this:

module.exports = {
  cacheHandler: require.resolve('next/dist/server/lib/incremental-cache/file-system-cache.js'),
  cacheMaxMemorySize: 0,

Needs tests!

[timfish]

Ok, so a bit of an update of where I've got to...

I've got this PR working in a demo app via yarn link:

...but in the e2e tests, import-in-the-middle is getting bundled which stops it from working. I'm using the serverExternalPackages config option (experimental.serverComponentsExternalPackages in < Next v15) which should be able to add import-in-the-middle to webpack externals but this doesn't appear to work in combination with pnpm when import-in-the-middle is a sub-dependency:

• mongodb still bundled in RSC when imported indirectly by another package of a pnpm monorepo vercel/next.js#49696

I've opened an issue to include import-in-the-middle in the next.js default server externals because it will never work when bundled but I kinda doubt this will actually fix the issue with pnpm 🤷‍♂️

• fix: Add import-in-the-middle to server-external-packages vercel/next.js#68937

[mydea]

@timfish is this still something you want to merge at some point, or can/should we close this?

[timfish]

Closing for now