Automatically exclude tunnel request from backend transactions

### Problem Statement

When setting up the `tunnel` option, users will often follow [the docs](https://docs.sentry.io/platforms/javascript/troubleshooting/#using-the-tunnel-option) and add a local API route to forward those envelopes.

These POST requests will usually get auto-instrumented on the backend and will potentially result in a transaction for every frontend envelope.

### Solution Brainstorm

Filter these out in some way?

If the `tunnel` option is set in the frontend, we could automatically set the `Content-Type` as `application/x-sentry-envelope`. This can be picked up in the http instrumentation and used to ignore these requests.

- Adding the `Content-Type` header will also fix a Svelte issue #16112