Support Merge for wide-column entities in the compaction logic (#10946)

Support Merge for wide-column entities in the compaction logic (#10946)

Summary:
The patch extends the compaction logic to handle `Merge`s in conjunction with wide-column entities. As usual, the merge operation is applied to the anonymous default column, and any other columns are unaffected.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10946

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D41233722

Pulled By: ltamasi

fbshipit-source-id: dfd9b1362222f01bafcecb139eb48480eb279fed