Advanced Support for Short-Lived Cryptographic Material - `flux push artifact` family

As mentioned in #5702, advanced features for the flux push artifact family of CLI commands are left out of the RFC scope, as they entail a specific flag API that applies only to these commands. The RFC discusses only the API fields and controller options that will apply to all the Flux controllers. The purpose of this issue is to close that gap, defining the scope and proposal for those features.

We will essentially capture the useful features already implemented in flux-mirror and implement them in the flux push artifact commands with a flag API that makes sense for these commands. Those include:

• GitHub/Forgejo Actions OIDC.
• OIDC tokens from environment variables (for the projected GitLab CI token).
• Kubernetes ServiceAccount token for generic container registries.
• SPIFFE features.

I will update this issue at some point to discuss the flag API.