Prepare for content-aware merges

[snazy]

Merging in Nessie is currently constrained by the fact that changes to the same content object on the references to be merged fail. There are options to mitigate the issue a little bit, by forcing the merge to let one of the changes survive, but this is not the ultimately satisfying solution.

Nessie needs a "more interactive" merge API that responds with the conflicting contents and lets callers chose a new conflict resolution type, namely to use a new content object supplied by the caller.

So the changes could:

• Add a new enum value APPLY to org.projectnessie.model.MergeBehavior
• Add three new fields of type Content to MergeKeyBehavior:
  • expectedSourceContent (expected content on the "merge from" reference)
  • expectedTargetContent (expected content on the "merge into" branch
  • resolvedContent (content to be eventually merged)
• Add two new fields to MergeResponse.ContentKeyDetails:
  • sourceContent (current content on the "merge from" reference)
  • targetContent (current content on the "merge into" branch

This gives callers, for example Iceberg, the ability to create a new table-metadata that is the result of the merge of both the source and target table-metadata.

Since this is a REST API change, at least the API change must be merged before calling REST API v2 "GA".