MINOR: acme: acme-vars allow to pass data to the dpapi sink

MINOR: acme: acme-vars allow to pass data to the dpapi sink

In the case of the dns-01 challenge, the agent that handles the
challenge might need some extra information which depends on the DNS
provider.

This patch introduces the "acme-vars" option in the acme section, which
allows to pass these data to the dpapi sink. The double quotes will be
escaped when printed in the sink.

Example:

    global
        setenv VAR1 'foobar"toto"'

    acme LE
        directory https://acme-staging-v02.api.letsencrypt.org/directory
        challenge DNS-01
        acme-vars "var1=${VAR1},var2=var2"

Would output:

    $ ( echo "@@1 show events dpapi -w -0"; cat - ) | socat /tmp/master.sock -  | cat -e
    <0>2025-09-18T17:53:58.831140+02:00 acme deploy foobpar.pem thumbprint gDvbPL3w4J4rxb8gj20mGEgtuicpvltnTl6j1kSZ3vQ$
    acme-vars "var1=foobar\"toto\",var2=var2"$
    {$
      "identifier": {$
        "type": "dns",$
        "value": "example.com"$
      },$
      "status": "pending",$
      "expires": "2025-09-25T14:41:57Z",$
      [...]