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

tests/unit/40_proxy.bats

@@ -51,7 +51,7 @@ assert_external_port() {
   assert_success
 
   for CID_FILE in $DOKKU_ROOT/$TEST_APP/CONTAINER.web.*; do
-    assert_external_port $(< $CID_FILE) success
+    assert_external_port $(< $CID_FILE) failure
   done
 
   run dokku proxy:enable $TEST_APP