feat: Add attachment support for Signal and WhatsApp channels

- Signal: Use base64Attachments field in signal-cli-rest-api
- WhatsApp: Implement Baileys attachment sending for images, videos, audio, and documents
- Both channels retrieve attachments from Zammad Store model
- Support multiple attachments per message

packages/zammad-addon-bridge/src/lib/cdr_signal_api.rb

@@ -41,6 +41,10 @@ class CdrSignalApi
       params[:conversationId] = options[:conversationId]
       options.delete(:conversationId)
     end
+    if options[:attachments]
+      params[:attachments] = options[:attachments]
+      options.delete(:attachments)
+    end
     post('send', params.merge(parse_hash(options)))
   end
 end