Add UPI to PayloadGenerator

[codebude]

Type of issue

[ ] Bug
[ ] Question (e.g. about handling/usage)
[X] Request for new feature/improvement

What to do?

UPI standard should be added to PayloadGenerator class.
Specifications can be found here:

• https://developers.google.com/pay/india/api/web/create-payment-method
• https://www.npci.org.in/PDF/npci/upi/circular/2017/Circular18_BankCompliances_to_enbaleUPIMerchantecosystem_0.pdf
• https://tools.ietf.org/html/rfc8905