Introduce abstraction for storing TUF private keys in cloud secret managers

[doanac]

Create an option for the update server to load TUF keys from something like AWS KMS or Google Cloud Secret Manager

[doanac]

The terraform logic we are doing to deploy uses AWS secret manager. However, secrets are pulled from the secret manager and then stored on disk. This might be sufficient for a v1.

@mike-scott - what do you think? Should we move this issue to v2 for now?

For reference:

• https://github.com/foundriesio/update-server/blob/terraform-contrib/contrib/terraform/aws/packer/files/fioserver-bootstrap.sh#L80
• https://github.com/foundriesio/update-server/blob/terraform-contrib/contrib/terraform/aws/scripts/init-secrets.sh