Add `cargo audit` CI target

## Brief feature description

Can be installed with `cargo install cargo-audit` and run with `cargo audit`.

It lists all known security advisories/warnings from all dependencies. The goal would be that iceorx2 does not depend on any package that has known security issues.

A CI target shall verify it. The problem is that a security issue of an unmaintained package may cause a lot of effort on our side to fix it. So it should be an optional target that does not block the CI but informs us so that we can take action.

Start by locating the repository's existing CI configuration and checking how Rust dependency checks are run. Add an optional CI target that runs cargo audit, reports known advisories without blocking CI, and confirms that the check is visible for follow-up.

Written by the indexing model from the issue text.