DEV: Introduce an array type for Chat contracts (#22278)

This small patch registers a new `ActiveModel` type: `array`.
It will split a string on `,` to create a new array. If the value is
already an array, nothing will happen and for all other types, it will
wrap the value in an array.

Here’s an example on an existing contract:
```ruby
  attribute :target_usernames

  before_validation do
    self.target_usernames =
      (
        if target_usernames.is_a?(String)
          target_usernames.split(",")
        else
          target_usernames
        end
      )
  end

  # can be rewritten as:

  attribute :target_usernames, :array
```

plugins/chat/lib/chat/types/array.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module Chat
+  module Types
+    class Array < ActiveModel::Type::Value
+      def serializable?(_)
+        false
+      end
+
+      def cast_value(value)
+        case value
+        when String
+          value.split(",")
+        else
+          ::Array.wrap(value)
+        end
+      end
+    end
+  end
+end