Summary:
Not a CMake expert, and the current CMake build support added by this PR is
unlikely the best way of doing it. Sending out the PR to demonstrate it
can work.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9214

Test Plan:
Will need to update https://github.com/ajkr/dedupfs with CMake build.
Also, PR https://github.com/facebook/rocksdb/issues/9170 and PR https://github.com/facebook/rocksdb/issues/9206 both include CMake support for their
plugins, and can be used as a proof of concept.

Reviewed By: ajkr

Differential Revision: D32738273

Pulled By: riversand963

fbshipit-source-id: da87fb4377c716bbbd577a69763b48d22483f845