Renamed payment send method; updated documentation;

Fix for PaymentSend endpoints; removed unused constants

Remove deprecated method; Extend testing; Add deep link method for sh…

…ort links

Added method for mobile friendly deep links; Improved handling of rec…

…eived headers in callbacks from Settle;

Update payment link template

Add shape validator; fix date for fingerprints

Merge branch 'main' of github.com:SettleAPI/settle-sdk-php

Add method for callback validation

Cleanup old methods, Add Payment Request link methods, Add doc blocks

Updates