Summary:
Together with the existing constructor,
`explicit WriteBatch(std::string&& rep)`, this enables transferring `WriteBatch` via its `std::string` representation. Associated info like KV checksums are dropped but the caller can use `WriteBatch::VerifyChecksum()` before taking ownership if needed.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11482

Reviewed By: cbi42

Differential Revision: D46233884

Pulled By: ajkr

fbshipit-source-id: 6bc64a6e75fb7bbf61d08c09520fc3705a7b44d8