Add user_data as new purpose on Files API

OpenAI now supports `user_data` as an allowable purpose; updating the library to permit it.
https://platform.openai.com/docs/api-reference/files/create#files-create-purpose

Author: jontec

lib/openai/files.rb

@@ -5,6 +5,7 @@ class Files
       batch
       fine-tune
       vision
+      user_data
     ].freeze
 
     def initialize(client:)