Introduce debug exporter to stream records out of a broker

[npepinpe]

Description

This PR introduces a new DebugExporter and its paired DebugReceiver to stream records out of a broker. The exporter is focused on correctness and simplicity, not performance.

As mentioned, it's made of two components, an exporter and a receiver. The receiver is an HTTP 1.1 server with a single endpoint, POST /records. The endpoint expects a list of records (for forwards-compatibility), all of which it assumes come from the same partition. It will then answer with a 200 and the highest known acknowledged position for that partition in the form of { "position": long }, or simply a 204 and no body if there is no known acknowledged position.

To auto acknowledge, you could simply immediately acknowledge Long.MAX_VALUE for all known partitions before starting your test.

The exporter will export each record as a single POST. This is inefficient, but extremely simple and accurate, as any failure immediately causes the record to be retried.

At the moment, the exporter is dynamically packaged as a standalone JAR and copied over to the container on demand. The alternative was packaging it separately and adding it to the resources, which is less than ideal (and also makes testing flakier, i.e. did you rebuild the JAR for the exporter based tests?). I will anyway propose to move the exporter to the Zeebe project itself, so I hope we can simply get rid of this limitation in the future.

Due to this, however, it means any work on the exporter should be done carefully. The class must remain standalone, and any custom types it requires must be inner classes (to simplify packaging logic).

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