[SPARK-49879][CORE] Move TransportCipherUtil to a separate file to eliminate Java compilation warnings

### What changes were proposed in this pull request?
Run `build/mvn clean install -pl common/network-common`, we can see the following compilation warnings:

```
[WARNING] [Warn] /Users/yangjie01/SourceCode/git/spark-maven/common/network-common/src/main/java/org/apache/spark/network/crypto/CtrTransportCipher.java:73:11:
auxiliary class TransportCipherUtil in /Users/yangjie01/SourceCode/git/spark-maven/common/network-common/src/main/java/org/apache/spark/network/crypto/TransportCipher.java should not be accessed from outside its own source file
[WARNING] [Warn] /Users/yangjie01/SourceCode/git/spark-maven/common/network-common/src/main/java/org/apache/spark/network/crypto/GcmTransportCipher.java:63:15:
auxiliary class TransportCipherUtil in /Users/yangjie01/SourceCode/git/spark-maven/common/network-common/src/main/java/org/apache/spark/network/crypto/TransportCipher.java should not be accessed from outside its own source file
```

So this pr moves `TransportCipherUtil` to a separate file to eliminate the aforementioned Java compilation warnings.

### Why are the changes needed?
Fix compilation warnings.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
- Pass GitHub Actions
- locally run `build/mvn clean install -pl common/network-common`, no longer have the aforementioned compilation warnings.

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #48352 from LuciferYang/Move-TransportCipherUtil.

Authored-by: yangjie01 <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: LuciferYang

common/network-common/src/main/java/org/apache/spark/network/crypto/TransportCipher.java

@@ -17,32 +17,12 @@
 
 package org.apache.spark.network.crypto;
 
-import com.google.common.annotations.VisibleForTesting;
-import com.google.crypto.tink.subtle.Hex;
-import com.google.crypto.tink.subtle.Hkdf;
 import io.netty.channel.Channel;
 
-import javax.crypto.spec.SecretKeySpec;
 import java.io.IOException;
-import java.nio.charset.StandardCharsets;
 import java.security.GeneralSecurityException;
 
 interface TransportCipher {
     String getKeyId() throws GeneralSecurityException;
     void addToChannel(Channel channel) throws IOException, GeneralSecurityException;
 }
-
-class TransportCipherUtil {
-    /*
-     * This method is used for testing to verify key derivation.
-     */
-    @VisibleForTesting
-    static String getKeyId(SecretKeySpec key) throws GeneralSecurityException {
-        byte[] keyIdBytes = Hkdf.computeHkdf("HmacSha256",
-                key.getEncoded(),
-                null,
-                "keyID".getBytes(StandardCharsets.UTF_8),
-                32);
-        return Hex.encode(keyIdBytes);
-    }
-}

common/network-common/src/main/java/org/apache/spark/network/crypto/TransportCipherUtil.java

@@ -0,0 +1,41 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.network.crypto;
+
+import java.nio.charset.StandardCharsets;
+import java.security.GeneralSecurityException;
+import javax.crypto.spec.SecretKeySpec;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.crypto.tink.subtle.Hex;
+import com.google.crypto.tink.subtle.Hkdf;
+
+class TransportCipherUtil {
+  /**
+   * This method is used for testing to verify key derivation.
+   */
+  @VisibleForTesting
+  static String getKeyId(SecretKeySpec key) throws GeneralSecurityException {
+    byte[] keyIdBytes = Hkdf.computeHkdf("HmacSha256",
+        key.getEncoded(),
+        null,
+        "keyID".getBytes(StandardCharsets.UTF_8),
+        32);
+    return Hex.encode(keyIdBytes);
+  }
+}