[Improvement] Optimize count operation for iceberg (#22923)

Iceberg has its own metadata information, which includes count statistics for table data. If the table does not contain equli'ty delete, we can get the count data of the current table directly from the count statistics.

be/src/vec/exec/scan/vscan_node.cpp

@@ -126,6 +126,11 @@ Status VScanNode::init(const TPlanNode& tnode, RuntimeState* state) {
     } else {
         _push_down_agg_type = TPushAggOp::type::NONE;
     }
+
+    if (tnode.__isset.push_down_count) {
+        _push_down_count = tnode.push_down_count;
+    }
+
     return Status::OK();
 }