[opt](balance) add config balance_slot_num_per_path (#19869)

Make balance_slot_num_per_path configurable.

fe/fe-common/src/main/java/org/apache/doris/common/Config.java

@@ -944,6 +944,10 @@ public class Config extends ConfigBase {
     @ConfField(mutable = true, masterOnly = true)
     public static int partition_rebalance_max_moves_num_per_selection = 10;
 
+    // 1 slot for reduce unnecessary balance task, provided a more accurate estimate of capacity
+    @ConfField(masterOnly = true, mutable = true)
+    public static int balance_slot_num_per_path = 1;
+
     // This threshold is to avoid piling up too many report task in FE, which may cause OOM exception.
     // In some large Doris cluster, eg: 100 Backends with ten million replicas, a tablet report may cost
     // several seconds after some modification of metadata(drop partition, etc..).