Integrate External Secrets Operator and HashiCorp Vault

[d3adb5]

Create a SecretStore or multiple such resources that allows syncing Kubernetes built-in Secrets with data stored in HashiCorp Vault.

The External Secrets Operator has been deployed already in c00ffb4, and is maintained through the Operator Lifecycle Manager. The CRDs should therefore be in place, we just need to configure Kubernetes authentication on Vault and add the necessary service accounts. This may or may not be done on a case by case basis to limit the secrets each service account can read.

Current known use cases:

• When integrating services with SSO, a client ID and client secret are made necessary and should be stored in a secret.
• Terraform is likely to make its way into this repository (it has been written and is in use), we can deploy Atlantis and mount things like AWS credentials through external secrets.
• Should this repository or any others turn private for whatever reason, GitHub credentials for Argo CD can be stored in Vault.