crimson: specialize fmt::formatter<>() for crimson types

crimson: specialize fmt::formatter<>() for crimson types

otherwise we'd have FTBFS like

/usr/include/fmt/core.h:1727:3: error: static_assert failed due to requirement 'formattable' "Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api$
  static_assert(
  ^
/usr/include/fmt/core.h:1853:23: note: in instantiation of function template specialization 'fmt::detail::make_arg<true, fmt::basic_format_context<fmt::appender, char>, fmt::detail::type::custom_type, crimson::os:$
        data_{detail::make_arg<

please note, delta_op_t is lifted out of the templated outer class
to avoid the headache of specialization of template of template in
another namespace.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>