Create API to get paid amount for proposal (public API)

[thanhnp95]

I have developed Decred’s finance report, which estimates the amount spent on proposals using metadata’s budget information.
Site: https://bisonexplorer.com/decred/finance-report

However, relying solely on estimated data does not accurately reflect the actual spending on proposals. Therefore, I created this issue in the hope of extending the Politeia API to retrieve data on the actual amounts paid for proposals. This enhancement would enable the development of more precise financial reports based on actual expenditure, providing the community with valuable insights.

API infos:
Input: Proposal token
Output: Payment times and amounts paid