Summary:
If Option.file_checksum_gen_factory is set, rocksdb generates the file checksum during flush and compaction based on the checksum generator created by the factory and store the checksum and function name in vstorage and Manifest.

This PR enable file checksum generation in SstFileWrite and store the checksum and checksum function name in the  ExternalSstFileInfo, such that application can use them for other purpose, for example, ingest the file checksum with files in IngestExternalFile().
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6859

Test Plan: add unit test and pass make asan_check.

Reviewed By: ajkr

Differential Revision: D21656247

Pulled By: zhichao-cao

fbshipit-source-id: 78a3570c76031d8832e3d2de3d6c79cdf2b675d0