util: add fmt::formatter for bool_class<Tag>

this change addresses the formatting with fmtlib v10, which stopped
creating the default-generated formatter even if FMT_DEPRECATED_OSTREAM
preprocess macro is defined. so to enable Seastar applications to
print bool_class<Tag> using {fmt} without defining the formatter
by themselves, we provide the formatter for this class in Seastar.

the operator<< for bool_class<Tag> is preserved for backward
compatibility. please note, {fmt} format a `bool` variable as "true" or
"false" based on its value, so we don't have to put "true" or "false"
explicitly as we do in the implementation of operator<<.

Refs scylladb/scylladb#13245

Signed-off-by: Kefu Chai <[email protected]>

Closes #2144

include/seastar/util/bool_class.hh

@@ -22,6 +22,7 @@
 #pragma once
 
 #include <ostream>
+#include <fmt/core.h>
 
 namespace seastar {
 
@@ -101,3 +102,12 @@ const bool_class<Tag> bool_class<Tag>::no { false };
 /// @}
 
 }
+
+template<typename Tag>
+struct fmt::formatter<seastar::bool_class<Tag>> {
+    constexpr auto parse(format_parse_context& ctx) { return ctx.begin(); }
+    template <typename FormatContext>
+    auto format(seastar::bool_class<Tag> v, FormatContext& ctx) const{
+        return fmt::format_to(ctx.out(), "{}", bool(v));
+    }
+};