[ADD] custom_vendor_portal: Enhance Vendor Portal with Purchase Order Creation

[nmis-odoo]

• Implemented vendor filtering by country, vendor, category, and product name.
• Added pagination support for better navigation of vendor products.
• Displayed product details with vendor pricing in the vendor portal.
• Introduced a Create Purchase button to open a modal for PO creation.
• Modal dynamically loads vendors for the selected product.
• Added a quantity input field for purchase orders.
• Implemented backend logic to create or merge draft purchase orders.
• Enhanced UI with proper sorting, filtering, and responsive design.
• Added JavaScript to handle modal interactions and success messages.

[robodoo]