Added parser for Linux kernel netfilter firewall log

[protohuf]

Adds a parser for Linux kernel netfilter firewall log messages. Applying this parser on the /var/log/kern.log will extract firewall logs

Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change
• Debug log output from testing the change

pipeline:
  inputs:
    - name: tail
      tag: firewall
      path: /var/log/kern.log
      parser: kmsg-netfilter-log
  outputs:
    - name: stdout

• Attached Valgrind output that shows no leaks or memory corruption was found

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.