multi: Inbound fees are retained when not provided

Fixes the problem that inbound base fee and fee rate are overwritten
with 0 if they are not specified in PolicyUpdateRequest. This ensures
backward compatibility with older rpc clients that do not yet support
the inbound feature.

lnrpc/lightning.proto

@@ -4369,6 +4369,16 @@ message FeeReportResponse {
     uint64 month_fee_sum = 4;
 }
 
+message InboundFee {
+    // The inbound base fee charged regardless of the number of milli-satoshis
+    // received in the channel. By default, only negative values are accepted.
+    int32 base_fee_msat = 1;
+
+    // The effective inbound fee rate in micro-satoshis (parts per million).
+    // By default, only negative values are accepted.
+    int32 fee_rate_ppm = 2;
+}
+
 message PolicyUpdateRequest {
     oneof scope {
         // If set, then this update applies to all currently active channels.
@@ -4402,8 +4412,9 @@ message PolicyUpdateRequest {
     // If true, min_htlc_msat is applied.
     bool min_htlc_msat_specified = 8;
 
-    int32 inbound_base_fee_msat = 10;
-    int32 inbound_fee_rate_ppm = 11;
+    // Optional inbound fee. If unset, the previously set value will be
+    // retained [EXPERIMENTAL].
+    InboundFee inbound_fee = 10;
 }
 
 enum UpdateFailure {