add end2end testing harness to capture-server crate

[xvello]

• Factor out server initialization in a serve function, that main now calls
• Create ServerHandle and EphemeralTopic test helpers, that setup a per-test output topic and per-test server on a random port, to ensure safe concurrency between tests. They implement automatic cleanup on Drop.
• Add two simple tests to try out the test harness
• Add a docker-compose with the proper startup healthcheck and an optional kafka-ui, but the tests work with the compose from posthog/posthog, to simplify dev workflows

Calling docker compose with the --wait argument makes it return only when the healthchecks are green, which should avoid flakes, but I'm starting the containers ASAP just to be sure.

TODO:

• redis isolation will be a bit more tricky. The redis-rs library does not support key prefixing, but we could implement it in our abstraction layer

[ellie]

Couple of comments, opinions not that strongly held