[Feature] Add an IPv6 on-link route to gateway

[mikucat0309]

Feature Request

Always add an IPv6 on-link static route to gateway that in nocloud network-config.

Description

Some cloud server providers assign an IPv6 prefix and an IPv6 gateway that is not within that prefix in nocloud network-config. As a result, there will be no IPv6 network connectivity after booting a Talos Linux nocloud ISO image into maintenance mode.

Example network-config

version: 2
ethernets:
    eth0:
        match:
            macaddress: '11:22:33:44:55:66'
        addresses:
            - 11.22.33.44/24
            - '1111:2222:3333:5678::1/64'
        gateway4: 11.22.33.1
        gateway6: '1111:2222:3333::1'
        nameservers:
            addresses:
                - 8.8.8.8
                - 8.8.4.4
                - '2001:4860:4860::8888'
                - '2001:4860:4860::8844'

Always add an IPv6 on-link static route to gateway can solve this problem.

Notes: I'm willing to implement this feature and open a pull request.