Add Assign Companies Feature for Customers

- Introduced a new endpoint to assign companies to a customer, enhancing the customer management capabilities.
- Implemented the AssignCompanies handler in the customer service layer, including validation and structured responses.
- Updated the routes to include the new PATCH /admin/v1/customers/{id}/companies endpoint, improving administrative functionalities.
- Enhanced API documentation with Swagger comments for the new endpoint, ensuring clarity for API consumers.

cmd/web/router/routes.go

@@ -82,6 +82,7 @@ func RegisterAdminRoutes(e *echo.Echo, userHandler *user.Handler, companyHandler
 		customersGP.DELETE("/:id", customerHandler.DeleteCustomer)
 		customersGP.PUT("/:id/status", customerHandler.UpdateStatus)
 		customersGP.PATCH("/:id/role", customerHandler.AssignRole)
+		customersGP.PATCH("/:id/companies", customerHandler.AssignCompanies)
 	}
 
 	// Admin Tenders Routes