Merge operator failed subcode (#11231)

Merge operator failed subcode (#11231)

Summary:
From HISTORY.md: Added a subcode of `Status::Corruption`, `Status::SubCode::kMergeOperatorFailed`, for users to identify corruption failures originating in the merge operator, as opposed to RocksDB's internally identified data corruptions.

This is a followup to https://github.com/facebook/rocksdb/issues/11092, where we gave users the ability to keep running a DB despite merge operator failing. Now that the DB keeps running despite such failures, they want to be able to distinguish such failures from real corruptions.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11231

Test Plan: updated unit test

Reviewed By: akankshamahajan15

Differential Revision: D43396607

Pulled By: ajkr

fbshipit-source-id: 17fbcc779ad724dafada8abd73efd38e1c5208b9