Add test utility that is able to intercept events

[lforst]

Problem Statement

One struggle we currently have in E2E and integration tests is that when the test suite (e.g. jest) and the testee (e.g. next.js server) run in different processes, we have a hard time asserting on outgoing requests to Sentry.

Solution Brainstorm

Build a utility (probably a package) that can boot up a server that can also act as a proxy to Sentry. The utility should also provide an interface to listen to particular requests/events.