Add advisory for capnp unsoundness

[shinmao]

The unsoundness issue has been verified and patched in the latest version of crate: capnproto/capnproto-rust#605

I would like to confirm for the maintainer that whether the advisory will break any normal use cases.

From my understanding, this will only alarm in the cargo audit so it might not be bothering. Also, the users of the crate should still be notified the potential UB can be triggered with the safe function in affected versions.

[djc]

Does this need to be updated to point to 0.25.0 as the fixing version?

[dwrensha]

0.24.0 fixes the issue, but at the cost of adding unsafe markers in generated code that gets included in downstream projects.

0.25.0 removes the need for that unsafe. I expect that skipping version 0.24.0 will be the smoothest upgrade path for most projects.

[shinmao]

Thanks @dwrensha for patch this issue. I think we can move forward to close this PR.