Allow additional DNS server per device

[b-m-f]

• add new attribute to the devices in the store
• Add input component/field (maybe a multi-select) to Device Component in expert mode that allows to specify additional DNS servers that device will have in its WireGuard configuration.
• Prefill the multi-select with DNS servers available on other devices, use a computed property and access the state to loop over the devices to get this data
• Use the new data in wireguard.js to add to the DNS config. This should be comma seperated
• write a test for this in wireguard.spec.js
• write an e2e test in the dashboard.spec.js

[b-m-f]

A zone that is meant for the additional DNS server must be provided to ignore it in CoreDNS