sweep+lnrpc: add new interface FeePreference

This commit adds a new interface `FeePreference` which makes it easier
to write unit tests and allows more customized implementation in
following commits.

lnrpc/walletrpc/walletkit_server.go

@@ -884,7 +884,13 @@ func (w *WalletKit) PendingSweeps(ctx context.Context,
 		broadcastAttempts := uint32(pendingInput.BroadcastAttempts)
 		nextBroadcastHeight := uint32(pendingInput.NextBroadcastHeight)
 
-		requestedFee := pendingInput.Params.Fee
+		feePref := pendingInput.Params.Fee
+		requestedFee, ok := feePref.(sweep.FeeEstimateInfo)
+		if !ok {
+			return nil, fmt.Errorf("unknown fee preference type: "+
+				"%v", feePref)
+		}
+
 		requestedFeeRate := uint64(requestedFee.FeeRate.FeePerVByte())
 
 		rpcPendingSweeps = append(rpcPendingSweeps, &PendingSweep{