Next.js managed tunnel on the tunnel route handler #25466

Next.js already has tunnelRoute. The plan is to land the opt-in route handler from #25456 first, then core (#25460), then rebuild the managed tunnel on top of that handler with createSentryTunnelRoute({ allowedDsns }) in place of our tunnelRoute.allowedDsns.

The generated route file stays user editable, and isTunnelRequest is exported so middleware can let tunnel requests through.

PR: #25424