Add option to use External secrets store operator to provide secrets to KBS

[bpradipt]

External Secrets Operator integrates external secret management systems like AWS Secrets Manager, HashiCorp Vault, Azure Key Vault, etc. The operator reads information from external APIs and automatically injects the values into a Kubernetes Secret.

https://external-secrets.io/latest/

This issue is to track the integration of Trustee operator with external secrets store

[bpradipt]

This will be mostly documentation.
cc @lmilleri