Merge pull request 'chore: change to discount_id to discount_code for buy asset req' (#19) from chore-change-buyreq-discount into v2

Reviewed-on: #19

wallet/v1/msg.proto

@@ -544,7 +544,7 @@ message BuyAssetReq {
   double amount_irt = 3;
   double amount_asset = 4;
   BuyAssetSide side = 5;
-  optional uint64 discount_id = 6;
+  optional string discount_code = 6;
   optional auth.v1.TFA tfa = 7; // This might be used in the future
 }