Added support for authentication methods

[whs]

This PR adds

• --auth / VAC_AUTH to configure authentication method. Default is token which is the same as existing behavior
• --auth-k8s-role / VAC_AUTH_K8S_ROLE to configure Kubernetes role nameee

At this time the K8S JWT path is not configurable

[Strangerxxx]

Hi @whs!

Looks like we both using the same features 😆

Please take a look on my PR #116, as more feature-rich implementation, including oidc and jwt token auth as well as many others (just like original vault cli does)

Thank you!

[whs]

Thanks for posting it here.

I'm not sure if the cli has a stable public API?

[Strangerxxx]

Actually, I'm not sure enough too 🤣

Seems like Vault CLI is using credential builtins.

They are publicly available, or at least can be imported =)
So I've took those and glued together using rewritten original vault login command code in my PR in order to achieve same features.

I think It would be way better to rewrite those builtins completely, but unfortunately it is more than I can handle on my own.

Can we cooperate on this feature, how do you think?