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 a formatter for data_value, but its
its operator<<() is preserved as we are still using the generic
homebrew formatter for formatting std::vector, which in turn uses
operator<< of the element type.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#16767