feat: allow users to bind to all interfaces when a proxy is enabled

- use a plugin trigger to see whether we should bind to all interfaces
- create a generic way of setting properties for a plugin
- migrate proxy-enabled to the new network property "bind-all-interfaces"
- add network:set subcommand

plugins/network/src/subcommands/set/set.go

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"flag"
+
+	common "github.com/dokku/dokku/plugins/common"
+	network "github.com/dokku/dokku/plugins/network"
+)
+
+// set or clear a network property for an app
+func main() {
+	flag.Parse()
+	appName := flag.Arg(1)
+	property := flag.Arg(2)
+	value := flag.Arg(3)
+
+	if property == "bind-all-interfaces" && value == "" {
+		value = "false"
+	}
+
+	common.CommandPropertySet("network", appName, property, value, network.ValidProperties)
+}