Ability to capture login PIN (`C_Login`)

Hi,

I'm trying out your excellent library and the one thing I'm missing is the ability to capture PIN as entered by the client (basically the `C_Login` function).

I'm currently using this workaround:

```rust
native_pkcs11_traits::register_backend(Box::new(Backend::default()));
unsafe {
info!("replacing login");
FUNC_LIST.C_Login = Some(C_Login_2);
FUNC_LIST.C_GetTokenInfo = Some(C_GetTokenInfo_2);
}
```

with:

```rust
#[unsafe(no_mangle)]
pub extern "C" fn C_Login_2(
_h_session: CK_SESSION_HANDLE,
user_type: CK_USER_TYPE,
p_pin: CK_UTF8CHAR_PTR,
ul_pin_len: CK_ULONG,
) -> CK_RV {
let pin = str::from_utf8(unsafe { slice::from_raw_parts(p_pin, ul_pin_len as usize) }).unwrap();

info!("got pin {user_type:?}, {pin}");
CKR_OK
}
```

which kind of works but sadly makes me deal with the low-level types.

Would support for capturing `C_Login` via high-level API (e.g. `native_pkcs11_traits::Backend`) be a good fit?

(I think it'd be possible to provide a default no-op function there so that the API stays stable).

Thanks for such a great library! 👋