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/cli/main.rb

@@ -189,6 +189,37 @@ class Kamal::Cli::Main < Kamal::Cli::Base
     end
   end
 
+  desc "upgrade", "Upgrade from Kamal 1.x to 2.0"
+  option :confirmed, aliases: "-y", type: :boolean, default: false, desc: "Proceed without confirmation question"
+  option :rolling, type: :boolean, default: false, desc: "Upgrade one host at a time"
+  def upgrade
+    confirming "This will replace Traefik with kamal-proxy and restart all accessories" do
+      with_lock do
+        if options[:rolling]
+          (KAMAL.hosts | KAMAL.accessory_hosts).each do |host|
+            KAMAL.with_specific_hosts(host) do
+              say "Upgrading #{host}...", :magenta
+              if KAMAL.hosts.include?(host)
+                invoke "kamal:cli:proxy:upgrade", [], options.merge(confirmed: true, rolling: false)
+                reset_invocation(Kamal::Cli::Proxy)
+              end
+              if KAMAL.accessory_hosts.include?(host)
+                invoke "kamal:cli:accessory:upgrade", [ "all" ], options.merge(confirmed: true, rolling: false)
+                reset_invocation(Kamal::Cli::Accessory)
+              end
+              say "Upgraded #{host}", :magenta
+            end
+          end
+        else
+          say "Upgrading all hosts...", :magenta
+          invoke "kamal:cli:proxy:upgrade", [], options.merge(confirmed: true)
+          invoke "kamal:cli:accessory:upgrade", [ "all" ], options.merge(confirmed: true)
+          say "Upgraded all hosts", :magenta
+        end
+      end
+    end
+  end
+
   desc "version", "Show Kamal version"
   def version
     puts Kamal::VERSION