before this change, we rely on the default-generated fmt::formatter created from operator<<, but fmt v10 dropped the default-generated formatter. in this change, we define formatters for `hints::host_filter`. its operator<< is preserved as it's still used by the homebrew generic formatter for vector<>, which is in turn used by db/config.cc. Refs #13245 Signed-off-by: Kefu Chai <kefu.chai@scylladb.com> Closes scylladb/scylladb#17347