Summary:
This patch is the first part of adding blob dump support. It only adds blob dump support to the dump command. A follow up patch will add blob dump support to the dump_live_files command.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9881

Reviewed By: ltamasi

Differential Revision: D35796731

Pulled By: jowlyzhang

fbshipit-source-id: 2cc5973b222d505a331ac7b969edcf992b47c5ee