security/acme-client: add Proxmox Backup Server

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Please add the same functionality for Proxmox BS as for Proxmox VE.

This is already available in the ACME project itself. https://github.com/acmesh-official/acme.sh/pull/6191 https://github.com/acmesh-official/acme.sh/blob/master/deploy/proxmoxbs.sh

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