[MINOR][YARN] Add YARN-specific credential providers in debug logging message

This PR adds a debugging log for YARN-specific credential providers which is loaded by service loader mechanism.

It took me a while to debug if it's actually loaded or not. I had to explicitly set the deprecated configuration and check if that's actually being loaded.

The change scope is manually tested. Logs are like:

```
Using the following builtin delegation token providers: hadoopfs, hive, hbase.
Using the following YARN-specific credential providers: yarn-test.
```

Author: hyukjinkwon <[email protected]>

Closes #21466 from HyukjinKwon/minor-log.

Change-Id: I18e2fb8eeb3289b148f24c47bb3130a560a881cf

Author: HyukjinKwon

core/src/main/scala/org/apache/spark/deploy/security/HadoopDelegationTokenManager.scala

@@ -32,7 +32,7 @@ import org.apache.spark.internal.Logging
  *
  * Also, each HadoopDelegationTokenProvider is controlled by
  * spark.security.credentials.{service}.enabled, and will not be loaded if this config is set to
- * false.  For example, Hive's delegation token provider [[HiveDelegationTokenProvider]] can be
+ * false. For example, Hive's delegation token provider [[HiveDelegationTokenProvider]] can be
  * enabled/disabled by the configuration spark.security.credentials.hive.enabled.
  *
  * @param sparkConf Spark configuration
@@ -52,7 +52,7 @@ private[spark] class HadoopDelegationTokenManager(
 
   // Maintain all the registered delegation token providers
   private val delegationTokenProviders = getDelegationTokenProviders
-  logDebug(s"Using the following delegation token providers: " +
+  logDebug("Using the following builtin delegation token providers: " +
     s"${delegationTokenProviders.keys.mkString(", ")}.")
 
   /** Construct a [[HadoopDelegationTokenManager]] for the default Hadoop filesystem */

resource-managers/yarn/src/main/scala/org/apache/spark/deploy/yarn/security/YARNHadoopDelegationTokenManager.scala

@@ -44,6 +44,10 @@ private[yarn] class YARNHadoopDelegationTokenManager(
 
   // public for testing
   val credentialProviders = getCredentialProviders
+  if (credentialProviders.nonEmpty) {
+    logDebug("Using the following YARN-specific credential providers: " +
+      s"${credentialProviders.keys.mkString(", ")}.")
+  }
 
   /**
    * Writes delegation tokens to creds.  Delegation tokens are fetched from all registered