Add packet sniffer feature

[WTPENGUIN]

Hello,

Please note that I am not fluent in English, so I used a translation tool to write this pull request. I apologize in advance for any awkward wording.

This contribution introduces a sniffer feature, inspired by the functionality provided in mavlink-router. The purpose is to enable a similar capability directly in mavp2p, allowing users to conveniently inspect MAVLink traffic without relying on external tools.

To use the sniffer feature, specify the desired system ID with the --sniffer-sysid startup option. This system ID will then be treated as a "sniffer." When enabled, mavp2p will attempt to forward all incoming packets to the designated sniffer. The default value is 0 (zero), which disables the feature. If forwarding is attempted back to the same sender (self-forwarding) or if the node with the specified sniffer system ID is not present on the network, a warning will be displayed and the forwarding will not be performed.

mavp2p --sniffer-sysid=30 udps:0.0.0.0:14550 # System ID 30 is treated as a 'sniffer'

I am not very familiar with the Go language, so some parts of my implementation may not fully follow common practices or might look unusual. I would greatly appreciate any feedback or suggestions for improvement.

Thank you for your consideration.