add vxlan interface plugin

[stefanpenner]

Part of a 2-PR stack: #1902 (this) vxlan interface plugin · #1903 network config overlay. Independent; better together.

Why

Today a vxlan can only be terminated on the box by creating the netdev out-of-band and adopting it through the phy plugin — FireRouter never manages its lifecycle. There is no native vxlan interface type.

How

Adds one, modeled on the vlan plugin: modprobe vxlan, create the link with ip link add … type vxlan …, delete it on flush, and re-apply when its underlay (intf) changes if that underlay is managed. Declared under interface.vxlan.<name>; with meta.type: lan it is monitored, DHCP-served, and firewalled like any LAN. Default MTU 1450 (1500 underlay − 50 overhead).

Config keys: vni (required), intf (underlay), local, remote/group, dstport (default 4789).

"interface": {
  "vxlan": {
    "vxlan99": {
      "enabled": true,
      "meta": { "type": "lan", "name": "exp" },
      "vni": 99,
      "intf": "br0",
      "local": "10.86.1.1",
      "remote": "10.86.1.32",
      "ipv4": "10.99.0.1/24"
    }
  }
}