Summary:
Add NewFileChecksumGenCrc32cFactory to file checksum public interface such that applications can use the build in crc32 checksum factory.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6688

Test Plan: pass make asan_check

Reviewed By: riversand963

Differential Revision: D21006859

Pulled By: zhichao-cao

fbshipit-source-id: ea8a45196a8b77c310728ab05f6cc0f49f3baef0