[terraform] configure intial sops setup

Summary:
This sets up sops for our self-host keyserver config.
secrets are stored in keyserver_secrets.json

Test Plan:
terraform apply and outputted secrets from keyserver_secrets.json like so:

output "my_secret" {
  value = nonsensitive(local.secrets["mariaDB"])
}

Depends on D12277

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12297