subprojects: add the foreign crate

subprojects: add the foreign crate

This is a cleaned up and separated version of the patches at
https://lore.kernel.org/all/20240701145853.1394967-4-pbonzini@redhat.com/
https://lore.kernel.org/all/20240701145853.1394967-5-pbonzini@redhat.com/

Its first user will be the Error bindings; for example a QEMU Error ** can be
converted to a Rust Option using

     unsafe { Option::<Error>::from_foreign(c_error) }

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>