feat: Add Supplemental Feeds feature with UI and backend support

- Implemented the main view for Supplemental Feeds, displaying clients with Merchant Account IDs and their associated feed files.
- Added styling for the feeds page and its components, including headers, empty states, and dropdown menus for syncing actions.
- Created backend logic to generate supplemental feeds for clients, including file handling and data sanitization.
- Integrated new routes and views for managing feeds, ensuring proper data retrieval and display.
- Updated navigation to include the new Supplemental Feeds section.
- Added necessary documentation for CRON job management related to feed generation.

feeds/supplemental_9.tsv

@@ -0,0 +1 @@
+id	title	description	google_product_category