This PR adds support for 2 MTA out modes : Direct-to-MX and SMTP-relay outbound delivery. Direct mode supports SOCKS5 proxies, and we bundle a new `src/socks-proxy` component to support it.
We also add an end-to-end self-check command plus scheduled health-check task with optional Prometheus metrics.
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Co-authored-by: Bastien Ogier <bastien.ogier@ext.anct.gouv.fr>
Co-authored-by: Stanislas Bruhiere <stanislas@bruhiere.fr>
