hold/release + discovery based routing

api/proto/common/gateway/v1/gateway.proto

@@ -45,6 +45,8 @@ enum RailOperation {
   RAIL_OPERATION_FEE = 4;
   RAIL_OPERATION_OBSERVE_CONFIRM = 5;
   RAIL_OPERATION_FX_CONVERT = 6;
+  RAIL_OPERATION_BLOCK = 7;
+  RAIL_OPERATION_RELEASE = 8;
 }
 
 // Limits in minor units, e.g. cents
@@ -124,6 +126,8 @@ message RailCapabilities {
   bool can_read_balance = 3;
   bool can_send_fee = 4;
   bool requires_observe_confirm = 5;
+  bool can_block = 6;
+  bool can_release = 7;
 }
 
 message LimitsOverride {