Upgrade commands for Kamal 1.x -> 2.0

Adds:
- `kamal upgrade` to upgrade all app hosts and accessory hosts
- `kamal proxy upgrade` to upgrade the proxy on all hosts
- `kamal accessory upgrade [name]` to upgrade accessories on all hosts

Upgrade takes rolling and confirmed options and calls `proxy upgrade`
and `accessory upgrade` in turn.

To just upgrade a single host add -h [host] to the command. But the
upgrade should run on all hosts, not just those running the proxy.

Calling upgrade on a host that has already been upgraded should work ok.

Upgrading hosts causes downtime but you can avoid if you run multiple
hosts by:
1. Implementing the pre-proxy-reboot and post-proxy-reboot hooks to
   remove the host from external load balancers
2. Running the upgrade with the --rolling option

**kamal proxy upgrade**
1. Creates a `kamal` network if required
2. Stops and removes the old proxy (whether Traefik or kamal-proxy)
3. Starts a kamal-proxy container in the `kamal` network
4. Reboots the app containers in the `kamal` network

**kamal accessory upgrade [name]**
1. Creates a `kamal` network if required
2. Reboots the accessory containers in the `kamal` network

A matching `downgrade` command will be added to Kamal 1.9.

lib/kamal/commander.rb

@@ -65,6 +65,13 @@ class Kamal::Commander
     end
   end
 
+  def with_specific_hosts(hosts)
+    original_hosts, self.specific_hosts = specific_hosts, hosts
+    yield
+  ensure
+    self.specific_hosts = original_hosts
+  end
+
   def accessory_names
     config.accessories&.collect(&:name) || []
   end