Add Cloud event serializer

[gklijs]

Having a cloud event serializer would make it easier to use this extension when there is an existing Kafka cluster used, where the chosen format on the wire is Cloud Events. For example to build a projection combining 'external' events from Kafka with 'internal' events from Axon Server, or to send certain events to Kafka for 'external' use.

Some things left to do

• Support mapping metadata key to extension name conversion, extension names can only use lowercase letters and digits. My idea is to add an optional String, String map to the builder, to do the conversion.
• Add autowire support, when the Cloudevent serializer is used, when should also change the Kafka config so the correct Kafka Serializer and Deserializer are used.

Optionally:

• Add an example project using cloud events, or add to the current one.
• [ ] Have a better way to deal with unsupported metadata like lists.

[gklijs]

I don't think we should/could take on the remaining point. I don't expect people to put Lists in the metadata, and it's something that's currently 'broken' for the DefaultKafkaMessageConverter. Some additional thing I'm not 100% happy with is that the dataContentType and dataSchema or lost when set, when converted to an event. I think that's fine as we don't have anything similar in Axon, although we could just put them in metadata.

[smcvb]

I don't think we should/could take on the remaining point. I don't expect people to put Lists in the metadata, and it's something that's currently 'broken' for the DefaultKafkaMessageConverter. Some additional thing I'm not 100% happy with is that the dataContentType and dataSchema or lost when set, when converted to an event. I think that's fine as we don't have anything similar in Axon, although we could just put them in metadata.

I think there's benefit on leaving that in the metadata. Assuming it's necessary to move back from Axon's Event format to Cloud Events, of course.

[gklijs]

Yes, it's also relatively easy to add. When going from Axon event -> cloud event we could have the default dataContentTypeSupplier and dataSchemaProvider get it from the metadata.

[smcvb]

Conceptually this looks fine to me. Just a bunch of JavaDoc comments, mainly.

[smcvb]

Two suggestions remaining. One JavaDoc, one on the source field on the Cloud Event. Otherwise, I think we're there.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

92.7% Coverage
0.0% Duplication

[smcvb]

My concerns have been addressed, hence I'm approving this pull request.