rust: pin-init: internal: pin_data: filter non-`#[cfg]` attr in generated code

rust: pin-init: internal: pin_data: filter non-`#[cfg]` attr in generated code

When using a macro with custom attributes in a `#[pin_data]` struct it
can mess up the generated code. The generated code needs nothing more than
the `#[cfg]` attribute, thus strip away all other attributes.

[ Rebased and updated to only include `#[cfg]` instead of both `#[cfg]` and
  `#[doc]`; doc is not needed for the generated hidden items. - Gary ]

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Co-developed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260527-pin-init-sync-v1-1-e20335ed2501@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>