Verify SDK behavior through E2E tests

[lforst]

Why

It's very valuable for us to have an E2E test setup that does the whole roundtrip from SDK to prod Sentry and back. It will enable us to not only verify that events aren't dropped somewhere but also verify any sort of post-processing that might happen before it hits the servers.

How

We're building on top of work done in #5506.

Separated into units of work:

• Define a standard for frontend E2E test applications (test(e2e): Add standard frontend test app specification #5874)
  • Keep in mind what we want to test with E2E tests! (errors, transactions, sessions)
  • Build a react application that follows the new standard
• Create behavior E2E tests for the new react app (test(e2e): Add behaviour test for Errors in standard React E2E tests application #5909, test(e2e): Add behaviour test for Transactions in standard React E2E tests application #5912)