Allow MTU configuration for Peers

[PapaZigE]

Is your feature request related to a problem? Please describe.
Yes. It's related to this Issue here: MTU configuration.

My peer server running services like Odoo, OneDev and Nextcloud fails to send SMTP due to MTU defaults being too low. They work properly for http requests, but ALL of them fail SMTP. After changing the interface MTU, everything worked again.

However, if I down and up netbird, it changes the MTU back to defaults. So even if I persist the changes at the server level, a Netbird restart will revert the changes. Note, All my services are in docker.

Describe the solution you'd like
Therefore, the ability to change the default config.json values and include an MTU option for peers would be extremely beneficial. Currently, if I add MTU to the config.json like this:

    "MTU": 1500,
    "WgIface": "wt0",
    "WgPort": 51820,
    "NetworkMonitor": null,
    "IFaceBlackList": [
        "wt0",
        "wt",
        "utun",
        "tun0",
        "zt",
        "ZeroTier",
        "wg",
        "ts",
        "Tailscale",
        "tailscale",
        "docker",
        "veth",
        "br-",
        "lo"
    ],

Netbird still does not pick it up. I can change the MTU with ip link but there's no way to persist it which is a problem because multiple services rely on the MTU setting as I have found out after a lot of troubleshooting. Although I can change it manually, it is not practical to instruct every peer that connects to change their interface MTU's. Peers are mixed between multiple clients like Windows, Mac, iPhone and Android. A default setting in the config and a way to have that config passed to the peers is the request.

Describe alternatives you've considered

• Manually setting interface MTU in config: does not work
• Manually setting interface MTU with ip link: works but does not persist on Netbird restart.