feat: add allowed_subnets for VPN relay/site-to-site configuration

[fherbert]

This adds support to add devices in site to site mode - ie as vpn relays.
Features:

• Extends device SQLModel to include allowed_subnets list
• Adds optional routed subnets (allowed_subnets) field to device configuration (both under user and admin view)

• If device has allowed_subnets configured:
  • The list of subnets will be appended on the server side peer allowed ip config

• Any traffic coming from these subnets will be forwarded to the users chain

• routes will be added to the routing table for each subnet

Happy to discuss this PR and can start conversation from here :-)