[USERSTORY] expand support to multiple KMS providers

[romdalf]

Is it linked to a user story? (use the "#" to tag the user story)
#2 #3 #4

What do we want to build?
An universal KMS provider plugin for Kubernetes (see https://kubernetes.io/docs/tasks/administer-cluster/kms-provider/) for more details.

Why do we want to build it?
To provide a Kubernetes native way to handle secrets, new and existing, without the need to deploy heavy components.

How do we want to design it?

• simple, light, and respectful of the Kubernetes KMS provider definition (see https://kubernetes.io/docs/tasks/administer-cluster/kms-provider/).
• based on the encryption envelope scheme mechanism
• should be transparent for the users
• a first version would support Hashicorp Vault as KMS to store the key used for encryption/decryption
• a first version would support self-hosted Kubernetes due to the need to restart the kube-apiserver with specific parameter "--encryption-provider-config"

[cannischan]

Review https://github.com/mozilla/sops to look at how to expand platform support

[cannischan]

• Draw architectural diagram @rovandep
• Estimate effort for Google KMS - prioritised
• Estimate effort for AWS - next item

[romdalf]

This userstory has been subject to our design meeting for v2 on Thursday June 9th 2022 leading the following milestone: https://github.com/ondat/trousseau/milestone/2

See: #50 for up-to-date details