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>
Closesscylladb/scylladb#17347
50964c423e