- update `prettify_xml` to strip XML declaration for more flexibility
- prepend XML declaration explicitly in Google Merchant feed generation
- adjust pagination `page_size` max limit to 128 for stricter validation
add feed generators for Google Merchant, Amazon Seller, Yandex Market, and Yandex Products. Includes a shared base class (`BaseFeedGenerator`) for common functionality, such as product retrieval and validation. Each generator supports format-specific output and handles platform-specific requirements.
This change simplifies the creation of product feeds for multiple marketplaces.
