Bring rdb to python3 and add it to CI-build or move it to a separate repo or deprecate it

[riversand963]

According to https://github.com/facebook/rocksdb/tree/main/tools/rdb#readme, rdb relies on Python2 for which support is no longer provided, even for security issues (https://www.python.org/doc/sunset-python-2/).

Currently, the tools/rdb is not compiled during daily development, making it easy to break. If the decision is to keep rdb in rocksdb main repo, we should include it in at least one CI build.

Two other options:

• Move it to a "plugin" repo outside the main repo
• Deprecate it

Looks like it is broken already because I don't think the following will compile: https://github.com/facebook/rocksdb/blob/main/tools/rdb/db_wrapper.cc#L8

// in rdb.cc and db_wrapper.cc
#include "db/_wrapper.h"