Support Kubernetes auth in hc-vault

[MiguelMartinho]

Hashicorp Vault can support authentication using a Kubernetes service account and role. Example: https://github.com/hashicorp/vault-examples/blob/main/examples/auth-methods/kubernetes/go/example.go

It would simplify the method to authenticate towards Vault if SOPS would support the use of a Kubernetes service account and role to access Hashicorp Vault (similar to the example above).