Make EventListener into a Customizable Class (#8473)

Make EventListener into a Customizable Class (#8473)

Summary:
- Added Type/CreateFromString
- Added ability to load EventListeners to DBOptions
- Since EventListeners did not previously have a Name(), defaulted to "".  If there is no name, the listener cannot be loaded from the ObjectRegistry.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8473

Reviewed By: zhichao-cao

Differential Revision: D29901488

Pulled By: mrambacher

fbshipit-source-id: 2d3a4aa6db1562ac03e7ad41b360e3521d486254