Add possibility to ignore specific advisories in audit test

[aghecenco]

We've recently hit an issue where an advisory was issued for an unmaintained crate (atty - advisory here) that several rust-vmm crates depend on. Since there's no apparent way to fix the issue (unresponsive maintainer), we need to allow for certain advisories such as this one to be skipped in the cargo audit test. Otherwise, CI will fail and block.

[andreeaflorescu]

This can also be disabled at the repository level by adding a file with the name audit.toml in .config/. An example is available here: https://github.com/rustsec/rustsec/blob/main/cargo-audit/audit.toml.example.

[andreeaflorescu]

We are enabling them per repo instead. Closing this issue.