[feature]: TrackPaymentRequest add payment_index for more specific payment tracking

[rsafier]

Is your feature request related to a problem? Please describe.

payment_hash is not unique, TrackPaymentRequest only lets you use this as the search criteria so cannot pick out specific payment record if there are multiple under same payment_hash

Describe the solution you'd like

Add payment_index to TrackPaymentRequest, if provided in request, use the more specific identifier to locate payment record else default to current behavior.

Describe alternatives you've considered

One can use ListPaymentsRequest with index offset set appropriately and requesting 1 result to get a rough non-streaming equivalent of the information.

Additional context