dhcp: Add priority option to dhcp.

Currently, we can not set the metric of routes in dhcp.
It's ok if there is only a network interface.

But if there are multiple network interfaces, and both have a default route,
We need to set the metric of the route to make the traffic
go through the correct network interface.

For host-local and static, we can set the metric with the route.priority option.
But there is no such option for dhcp.

Signed-off-by: Songmin Li <lisongmin@protonmail.com>

plugins/ipam/dhcp/main.go

@@ -51,6 +51,8 @@ type IPAMConfig struct {
 	// To override default requesting fields, set `skipDefault` to `false`.
 	// If an field is not optional, but the server failed to provide it, error will be raised.
 	RequestOptions []RequestOption `json:"request"`
+	// The metric of routes
+	Priority int `json:"priority,omitempty"`
 }
 
 // DHCPOption represents a DHCP option. It can be a number, or a string defined in manual dhcp-options(5).