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 db::seed_provider_type.
please note, we are still formatting vector<db::seed_provider_type>
with the helper provided by seastar/core/sstring.hh, which uses
operator<<() to print the elements in the vector being printed.
so we have to keep the operator<< formatter before disabling
the generic formatter for vector<T>.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#16138
6749d963ed