add support for docker compose secrets

[KiaraGrouwstra]

enables using docker compose secrets from arion, which includes:

• top-level secrets element defining the secrets to be used for the below two use-cases, exposing them at /run/secrets/<secret_name>. comes in flavors file vs environment.
• run-time: services top-level secrets element
• build time: build secrets (supported in docker compose tho not docker stack, to be mounted in the Dockerfile like RUN --mount=type=secret,id=<secret_name> ...)

unlike #52, i did not so far add support for their long syntax, which despite the confusing documentation appears limited to Docker Swarm, in my understanding currently limiting its use in Arion.