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
* operation::either_of<Ops...>
* operation::exceptional_result<Op>
* operation::completion<Op>
* operation::invocable<Op>
and drop their operator<<:s.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>