Update RocksDB submodule commit pointer to v6.20.3
Integrate building RocksDB plugins into RocksDB Storage Engine CMakeLists.txt
Include building ZenFS plugin (https://github.com/westerndigitalcorporation/zenfs)
Currently any version that includes fs: use read fd when reading zone report from ZNS device (commit 225215f3f384d4abd0a526b936ed8fe9540b4ae3) should suffice.
Expose --rocksdb-fs-uri parameter for specifying custom file systems