fix(wallet): fix request export order

[olaszakos]

• implements the sort_by optional field for GetNextApprovableRequestInput to be able to affect the order it will produce the next approvable request
• removes the extra sorting on the frontend for csv export generation that will put the results in reverse order
• bug fix: list_requests with only_approvable = true could return non-pending requests for users who could approve if the request was still pending

Manual testing findings:

1. Uploaded CSV file for batch transfers. The transfers are created from the frontend 10 in parallel at a time. The creation order inside a batch of 10 is random. The batch transfer feature therefore will create a list of transfers that dont keep the ordering of the original CSV input file.
2. Exporting the created transfers will produce a list that is sorted from most recent to oldest. This is in line with the ordering of the data table on the frontend.
3. Using the load-next button will enumerate the pending requests in the same order as in the exported CSV file. However if you start approving from the middle of the list, the next approvable request will jump back to the first one in the list.