feat: enhence log format

[sigoden]

support json log-format

$ dufs --log-format '{"time":"$time_local","addr":"$remote_addr","uri":"$request_uri", "method":"$request_method","status":$status}'

{"time":"2026-04-25T20:28:03+08:00","addr":"192.168.8.8","uri":"/", "method":"GET","status":200}

default text log-format

- $remote_addr "$request" $status
+ $time_iso8601 $log_level - $remote_addr "$request" $status

close #671