Add payment index to payviapaymentrequest and subscribetopayviarequest

[niteshbalusu11]

Fixed, comments, typescript and test cases.

[alexbosworth]

I added a comment on the attribute doc description, generally in those i try to kind of restate the field but in a different way than the actual naming

So I think this should be lining up with the offset number you'd see in the list of total payments