support compacted event types (#154) via spring beans

[ePaul]

This is another implementation of #154, where the user can define the compaction key extractors as spring beans, instead of using a register function on the event log writer (as was done in #158).

This seems to be a more spring-y way of doing this.

Doing this also gave the need/opportunity to rewrite the internal logic of finding the right extractor.

Opinions please: is this better or worse than the way in #158?

(Most of the commits are the same as in #158, the last ones are the new ones.)

For the change log:

---

• 164: support compacted event types (via spring beans)

  You now can fire events for a compacted event type, by adding a CompactionKeyExtractor spring bean.

[ePaul]

I plan to make two variant release candidate builds with both options, so I can try this out in an actual project where the need for this arose. (But I'm not sure whether I can get to this this week.)

[fbrns]

👍

[ePaul]

:+1: