We need to add the ability for the engine to look up environment variables and set them up as parameters or secrets in application services. An environment variable in a service should address the following: - An environment variable with a hardcoded value (eg. `PORT=4000`) - An environment variable that proxies an environment variable from the host OS (eg. `PORT=$STACKMATE_PORT`) - An environment variable that is constructed based on values from other services (eg. `DATABASE_URL=postgresql://<db user name>:<db password>@<db host name>/<db name>?schema=public` We need to: - Create placeholders for services - Expose the placeholders in JSON schema - Add a custom validator that validates placeholders in the config file