Bring RocksDB to PostgreSQL as an extension. It is the first foreign data wrapper (FDW) that introduces LSM-tree into PostgreSQL. The underneath storage engine can be RocksDB. The FDW also serves for VidarDB engine, a versatile storage engine for various workloads. See the link for more info about VidarDB engine.
Why do you think that https://github.com/ibarwick/firebird_fdw is a good alternative to pgrocks-fdw