Further support for SOPS/encryption, better documentation

[tschale]

Currently we already support PGP with SOPS in the form of the helm secrets plugin, however that is not clearly documented. You kind of have to know what to do in order to achieve it. Maybe I'll find some time to make small improvements to that.

We should discuss the further support for other SOPS encryption keys and for SOPS in general. We spoke about following points:

• direct support of SOPS
• support for git-crypt
• support for AWS KMS

SOPS further supports following further methods:

• GCP KMS
• Azure Key Vault
• Hashicorp Vault
• age (which is recommended over PGP by SOPS)

What do you think about which methods we should support out of the gate and in which priority?