defender: implement logging of events and bans

[Anthrazz]

This PR implements optional log messages for defender events and the final ban of IPs. They can be enabled with the new config parameter log_events: true in the defender configuration struct.

Log Messages for invalid user and user with wrong credentials look like this:

{"level":"info","time":"2023-12-19T10:04:39.093","sender":"defender","client_ip":"2a01:4f8::1","protocol":"FTP","event":"UserNotFound","increase_score_by":2,"score":2}
{"level":"info","time":"2023-12-19T10:04:45.360","sender":"defender","client_ip":"2a01:4f8::1","protocol":"FTP","event":"LoginFailed","increase_score_by":1,"score":3}

Log Messages when the score threshold is reached and the IP is banned:

{"level":"info","time":"2023-12-19T10:07:53.768","sender":"defender","client_ip":"2a01:4f8::1","protocol":"FTP","event":"UserNotFound","increase_score_by":2,"score":15}
{"level":"info","time":"2023-12-19T10:07:53.768","sender":"defender","client_ip":"2a01:4f8::1","protocol":"FTP","event":"banned"}

[drakkan]

Hello,

thanks for this contribution. I'm not sure we need yet another settings here. We could log events at debug level and bans at info level.

I'm also curious how SFTPGo is being used or planned to be used at Hetzner

[Anthrazz]

Hello,

Yes, that would work for us too. I have made the necessary changes to the PR.

We are currently evaluating SFTPGo as a service for our Storage Boxes.

[drakkan]

Thank you.

Before including SFTPGo in your Storage Boxes, please note that we have registered the SFTPGo trademark and no longer want to allow unauthorized distribution of SFTPGo. Thanks for understanding

[drakkan]

Hello,

thank you again for this contribution. We have recently updated our contribution policy and we now require signing a CLA. Agreeing to the CLA explicitly states that you are entitled to provide a contribution and that you cannot withdraw permission to use your contribution at a later date. This removes any ambiguities or uncertainties.

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