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 cql3::prepared_cache_key_type
and cql3::prepared_cache_key_type::cache_key_type, and remove
their operator<<().
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#16901