Add kamal secrets print for secret debugging

Dotenv's variable substitution doesn't work the same way as commands run
in the shell. It needs values to be escaped.

```sh
$ cat /tmp/env
SECRETS=$(cat /tmp/json)
SECRETS2=$(echo $SECRETS | jq)
$ cat /tmp/json
\{\ \"foo\"\ :\ \"bar\" \}
$ SECRETS=$(cat /tmp/json)
$ SECRETS2=$(echo $SECRETS | jq)
jq: parse error: Invalid numeric literal at line 1, column 2
$ ruby -e 'require "dotenv"; puts Dotenv.parse("/tmp/env")["SECRETS2"]'
{
  "foo": "bar"
}
```

Since you then can't use the shell to debug, `kamal secrets print` will
allow you to see what the secrets will be set to.

lib/kamal/cli/secrets.rb

@@ -21,6 +21,13 @@ class Kamal::Cli::Secrets < Kamal::Cli::Base
     return_or_puts value, inline: options[:inline]
   end
 
+  desc "print", "Print the secrets (for debugging)"
+  def print
+    KAMAL.config.secrets.to_h.each do |key, value|
+      puts "#{key}=#{value}"
+    end
+  end
+
   private
     def adapter(adapter)
       Kamal::Secrets::Adapters.lookup(adapter)