[fix](iceberg)Fix the thread pool issue used for commit. (#51508)

### What problem does this PR solve?

Problem Summary:

When Iceberg generates a new snapshot, it performs a merge operation
based on the previous snapshot. This operation reads manifest files, and
the file reading process uses a global thread pool. However, users may
have their own authentication information, which requires the use of
doAs to ensure context. Therefore, the thread pool provided by Iceberg
cannot be used.

### Release note

None

Author: wuwenchi

fe/fe-core/src/main/java/org/apache/doris/datasource/ExternalCatalog.java

@@ -1170,4 +1170,8 @@ public void setAutoAnalyzePolicy(String dbName, String tableName, String policy)
             tableAutoAnalyzePolicy.put(key, policy);
         }
     }
+
+    public ThreadPoolExecutor getThreadPoolExecutor() {
+        return threadPoolWithPreAuth;
+    }
 }

fe/fe-core/src/main/java/org/apache/doris/datasource/iceberg/IcebergMetadataOps.java

@@ -49,6 +49,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.concurrent.ThreadPoolExecutor;
 import java.util.stream.Collectors;
 
 public class IcebergMetadataOps implements ExternalMetadataOps {
@@ -291,4 +292,8 @@ private Namespace getNamespace(String dbName) {
     private Namespace getNamespace() {
         return externalCatalogName.map(Namespace::of).orElseGet(() -> Namespace.empty());
     }
+
+    public ThreadPoolExecutor getThreadPoolWithPreAuth() {
+        return dorisCatalog.getThreadPoolExecutor();
+    }
 }

fe/fe-core/src/main/java/org/apache/doris/datasource/iceberg/IcebergTransaction.java

@@ -155,7 +155,7 @@ private synchronized Table getNativeTable(SimpleTableInfo tableInfo) {
 
     private void commitAppendTxn(Table table, List<WriteResult> pendingResults) {
         // commit append files.
-        AppendFiles appendFiles = table.newAppend();
+        AppendFiles appendFiles = table.newAppend().scanManifestsWith(ops.getThreadPoolWithPreAuth());
         for (WriteResult result : pendingResults) {
             Preconditions.checkState(result.referencedDataFiles().length == 0,
                     "Should have no referenced data files for append.");
@@ -171,7 +171,7 @@ private void commitReplaceTxn(Table table, List<WriteResult> pendingResults) {
             // 1. if dst_tb is a partitioned table, it will return directly.
             // 2. if dst_tb is an unpartitioned table, the `dst_tb` table will be emptied.
             if (!table.spec().isPartitioned()) {
-                OverwriteFiles overwriteFiles = table.newOverwrite();
+                OverwriteFiles overwriteFiles = table.newOverwrite().scanManifestsWith(ops.getThreadPoolWithPreAuth());
                 try (CloseableIterable<FileScanTask> fileScanTasks = table.newScan().planFiles()) {
                     fileScanTasks.forEach(f -> overwriteFiles.deleteFile(f.file()));
                 } catch (IOException e) {