[Feature Request, maltrail.conf] Support of LOCAL_LOG_FORMAT variable

[MikhailKasimov]

Hello!

Deal: To add support of LOCAL_LOG_FORMAT variable for /maltrail.conf file. Inspirated by MikhailKasimov/maltrail-wazuh-decoder-and-rules#2 report. Would give a possibility to get info by reading respective log file(-s), especially can be useful for structured form of log-files.

Syntax: LOCAL_LOG_FORMAT text|json.

LOCAL_LOG_FORMAT text -- saving logs to /var/log/maltrail/ in plain-text format.
LOCAL_LOG_FORMAT json -- saving logs to /var/log/maltrail/ in json format.