Display __consumer_offsets contents

[timtebeek]

Fixes #654

[timtebeek]

There's still some awkward formatting of kafka-connect offset messages due to the metadata within there, but overal the messages are now readable and can be used for debugging, which was not possible before.

[tchiotludo]

Seems to works well thanks 👍

[tchiotludo]

also added __transaction_state topic since we are loading the class needed :=)

[timtebeek]

Only realized later that it might be better to show a JSON serialized object for the group meta data value rather than the toString of the same object. Since the UI expects and does syntax highlighting for JSON. Would you agree and like to see that changed?

[tchiotludo]

Yes it will be better 😊

[timtebeek]

Hmm; the returned OffsetAndMetadata class does not expose any getters for the JSON serializer to use, so you'd see an empty Object. We could:

1. Leave it as is
2. Map OffsetAndMetadata fields into a Map
3. Map OffsetAndMetadata to a DTO with getters

I'd lean towards option 1 or 2, especially since this is all quite exceptional handling for special topics.

[tchiotludo]

For transactions topics, I've choose the Map options