MINOR: acme: provider-name for dpapi sink

MINOR: acme: provider-name for dpapi sink

Like "acme-vars", the "provider-name" in the acme section is used in
case of DNS-01 challenge and is sent to the dpapi sink.

This is used to pass the name of a DNS provider in order to chose the
DNS API to use.

This patch implements the cfg_parse_acme_vars_provider() which parses
either acme-vars or provider-name options and escape their strings.

Example:

     $ ( 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"$
     provider-name "godaddy"$
     {$
       "identifier": {$
         "type": "dns",$
         "value": "example.com"$
       },$
       "status": "pending",$
       "expires": "2025-09-25T14:41:57Z",$
       [...]