Merge pull request 'chore: convert side array to single value' (#23) from chore-market-filter-side into v2

Reviewed-on: #23

market/v1/msg.proto

@@ -95,7 +95,7 @@ message OrderListFilter {
   optional auth.v1.InternalIAM iam = 1;
   base.v1.BaseQueryParam base = 2;
   optional string search = 3;
-  repeated MarketOrderSide side = 4;
+  optional MarketOrderSide side = 4;
   repeated MarketOrderStatus status = 5;
   repeated string from = 6;
   repeated string to = 7;