Add inventory management features including Sales, Stock Returns, and Suppliers

- Implement Sale and SaleItem models for tracking sales transactions.
- Create StockReturn model to manage stock adjustments for various reasons.
- Introduce Supplier model to represent suppliers of products.
- Develop InventoryRepository for CRUD operations on products, suppliers, sales, and stock returns.
- Implement InventoryService to handle business logic for inventory management.
- Update TenantDbContext to include new models and relationships.
- Enhance ProfitLossRepository to account for sales, purchases, and stock returns in profit and loss calculations.
- Modify Program.cs to register new inventory-related services.

Interfaces/Services/IInventoryService.cs

@@ -0,0 +1,26 @@
+namespace Fitness_core_BE.Interfaces.Services
+{
+    public interface IInventoryService
+    {
+        Task<object> GetProducts(object payload, HttpContext httpContext);
+        Task<object> GetProductById(object payload, HttpContext httpContext);
+        Task<object> CreateProduct(object payload, HttpContext httpContext);
+        Task<object> UpdateProduct(object payload, HttpContext httpContext);
+        Task<object> UploadProductImage(Guid productId, IFormFile image, HttpContext httpContext);
+
+        Task<object> GetSuppliers(object payload, HttpContext httpContext);
+        Task<object> CreateSupplier(object payload, HttpContext httpContext);
+
+        Task<object> CreateGrn(object payload, HttpContext httpContext);
+        Task<object> GetGrns(object payload, HttpContext httpContext);
+        Task<object> GetGrnById(object payload, HttpContext httpContext);
+
+        Task<object> CreateSale(object payload, HttpContext httpContext);
+        Task<object> GetSales(object payload, HttpContext httpContext);
+        Task<object> GetSaleById(object payload, HttpContext httpContext);
+
+        Task<object> CreateStockReturn(object payload, HttpContext httpContext);
+        Task<object> GetStockReturns(object payload, HttpContext httpContext);
+        Task<object> GetStockReturnById(object payload, HttpContext httpContext);
+    }
+}