ci(audit): add new nightly cargo audit job

[oleonardolima]

Description

As I was going through the recent audit issues in bdk and bdk_wallet I noticed this nightly job was missing here.

I've added the new audit.yml job in CI and also added the .cargo/audit.toml in case we need to ignore any unapplicable advisories in the future.

Changelog notice

### Added

- ci(audit): add new nightly `cargo audit` job
- chore: add new `.cargo/audit.toml`

Checklists

All Submissions:

• I followed the contribution guidelines

[luisschwab]

We don't commit the lockfile (yet). This can be added once the switch to cargo-rbmt is done here and we start comitting Cargo-minimal.lock and Cargo-recent.lock.