CreateConnection function establishes a connection to the PostgreSQL database.

CreateStock handles the HTTP POST request to create a new stock.

DeleteStockById handles the HTTP DELETE request to delete a stock by ID.

GetStockById handles the HTTP GET request to retrieve a stock by ID.

GetStocks handles the HTTP GET request to retrieve all stocks.

UpdateStockById handles the HTTP PUT request to update a stock by ID.

Response struct represents the structure for API responses.