Monitor DNS config files

[varunsh-coder]

@h0x0er please take this up.

We need to monitor the DNS config files for changes by adding the path to the process monitor.
These are the two files:
https://github.com/step-security/agent/blob/main/dnsconfig.go#L21-L22

If they are changed, we need to log that.

This is how you can add more files to be monitored:
https://github.com/step-security/agent/blob/main/procmon_linux.go#L62