Align e2e testing relay mock servers

### Description

There's currently multiple things in use when it comes to asserting envelopes make it out of the SDK and into relay.

We have:
- https://github.com/getsentry/sentry-java/pull/3811/files#diff-068624ded3e0ed8745999da8a9bccf9894c94522339169bad2e73ffaa7ca0465
- https://github.com/getsentry/sentry-java/blob/main/test/system-test-sentry-server.py
- https://github.com/getsentry/sentry-java/blob/main/sentry-android-integration-tests/sentry-uitest-android/src/androidTest/java/io/sentry/uitest/android/mockservers/MockRelay.kt

We should consider merging these.

Note: These might also be in use on other repos, e.g. SAGP