crimson/net: print shared_ptr using fmt::ptr()

crimson/net: print shared_ptr using fmt::ptr()

in {fmt} v9, only `void*` pointers can be printed directly. when it
comes to typed pointer or smart pointers, we need to print them using
`fmt::ptr()`. in this change, an overload is added so we can print
`seastar::shared_ptr<>` via `fmt::ptr()` as well, just like how we
print `std::shared_ptr<>`.

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