Make examples work with ambient identities

[jku]

It would be good to demonstrate how to use ambient identities with sigstore-rs.

• I've created https://crates.io/crates/ci-id. It's a tiny project that essentially copies the setup sigstore-python has with id
• I also have a small patch to sigstore-rs examples and tests to actually use it

This would add a few lines of code into the example and would not require new dependency (ci-id would be a dev-dependency though).

This depends on at least #413