in to_string.hh, we define the specialization of
`fmt::formatter<std::optional<T>>`, which is available in {fmt} v10
and up. to avoid conditionally including `utils/to_string.hh` and
`fmt/std.h` in all source files formatting `std::optional<T>` using
{fmt}, let's include `fmt/std.h` if {fmt}'s verison is greater or equal
to 10. in future, we should drop the specialization and use `fmt/std.h`
directly.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes scylladb/scylladb#18325
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