hc-vault: Support for kubernetes auth

[JuniorJPDJ]

There are code examples here: https://developer.hashicorp.com/vault/docs/auth/kubernetes#code-example

argocd-vault-plugin can already use it and it would be the best possible KMS alternative for on-premise usage
https://argocd-vault-plugin.readthedocs.io/en/stable/backends/#kubernetes-authentication

I personally prefer sops with helm-secrets than AVP as it's visible in git tree when secret changes and you can actually follow the changes later. Not talking about encrypting with spare key (eg. GPG) for some disaster case.

This allows usage of sops with helm-secrets in argocd-repo-server without providing any token to HC Vault, just using ServiceAccount.

Probably more usecases can be found, but IMO this one is major and it's actually my usecase.

[JuniorJPDJ]

As a workaround I created simple helm wrapper shell script which I ship with custom argocd image used as argocd-repo-server: https://github.com/JuniorJPDJ/argocd-helm-secrets/blob/master/helm-vault-k8s-auth-wrapper.sh

If someone needs similar solution feel free to use it.