Mark Payouts as Paid (#2539)

* Mark Payouts as Paid

This PR allows users to mark payouts as paid manually through the UI  and through the API. It also sets up the payout proof system to be able store a manual proof that will in a later PR allow you to specify a proof of payment (link or text)

* add docs, test and greenfield client

* remove extra docs stuff

* Update BTCPayServer.Tests/GreenfieldAPITests.cs

Co-authored-by: britttttk <39231115+britttttk@users.noreply.github.com>

* clean up pull payment/payouts fetch code

* Ensure payoutis are retrieved with pull payment

Co-authored-by: britttttk <39231115+britttttk@users.noreply.github.com>

BTCPayServer/Controllers/PullPaymentController.cs

@@ -42,6 +42,7 @@ namespace BTCPayServer.Controllers
             _serializerSettings = serializerSettings;
             _payoutHandlers = payoutHandlers;
         }
+        
         [Route("pull-payments/{pullPaymentId}")]
         public async Task<IActionResult> ViewPullPayment(string pullPaymentId)
         {