Send TaxCloud capture on invoice pay instead of order place

[azane-code]

Added configurable "When to send order to TaxCloud" setting

• Added admin option: On order creation, On payment, or On shipment (default: On order creation).
• Registered observer for sales_order_place_after, sales_order_invoice_pay, and sales_order_shipment_save_after; run authorizeCapture only when the current event matches the selected option.
• Added Model/Config/Source/CaptureTrigger and default in etc/config.xml.
• Only send on first invoice pay or first shipment when those options are selected.
• Update README