[SPARK-49791][SQL] Make DelegatingCatalogExtension more extendable

### What changes were proposed in this pull request?

This PR updates `DelegatingCatalogExtension` so that it's more extendable
- `initialize` becomes not final, so that sub-classes can overwrite it
- `delegate` becomes `protected`, so that sub-classes can access it

In addition, this PR fixes a mistake that `DelegatingCatalogExtension` is just a convenient default implementation, it's actually the `CatalogExtension` interface that indicates this catalog implementation will delegate requests to the Spark session catalog. apache#47724 should use `CatalogExtension` instead.

### Why are the changes needed?

Unblock the Iceberg extension.

### Does this PR introduce _any_ user-facing change?

no

### How was this patch tested?

existing tests

### Was this patch authored or co-authored using generative AI tooling?

no

Closes apache#48257 from cloud-fan/catalog.

Lead-authored-by: Wenchen Fan <wenchen@databricks.com>
Co-authored-by: Wenchen Fan <cloud0fan@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

sql/catalyst/src/main/java/org/apache/spark/sql/connector/catalog/DelegatingCatalogExtension.java

@@ -38,7 +38,7 @@
 @Evolving
 public abstract class DelegatingCatalogExtension implements CatalogExtension {
 
-  private CatalogPlugin delegate;
+  protected CatalogPlugin delegate;
 
   @Override
   public final void setDelegateCatalog(CatalogPlugin delegate) {
@@ -51,7 +51,7 @@ public String name() {
   }
 
   @Override
-  public final void initialize(String name, CaseInsensitiveStringMap options) {}
+  public void initialize(String name, CaseInsensitiveStringMap options) {}
 
   @Override
   public Set<TableCatalogCapability> capabilities() {

sql/core/src/main/scala/org/apache/spark/sql/catalyst/analysis/ResolveSessionCatalog.scala

@@ -28,7 +28,7 @@ import org.apache.spark.sql.catalyst.plans.logical._
 import org.apache.spark.sql.catalyst.rules.Rule
 import org.apache.spark.sql.catalyst.util.{quoteIfNeeded, toPrettySQL, ResolveDefaultColumns => DefaultCols}
 import org.apache.spark.sql.catalyst.util.ResolveDefaultColumns._
-import org.apache.spark.sql.connector.catalog.{CatalogManager, CatalogPlugin, CatalogV2Util, DelegatingCatalogExtension, LookupCatalog, SupportsNamespaces, V1Table}
+import org.apache.spark.sql.connector.catalog.{CatalogExtension, CatalogManager, CatalogPlugin, CatalogV2Util, LookupCatalog, SupportsNamespaces, V1Table}
 import org.apache.spark.sql.connector.expressions.Transform
 import org.apache.spark.sql.errors.QueryCompilationErrors
 import org.apache.spark.sql.execution.command._
@@ -706,6 +706,6 @@ class ResolveSessionCatalog(val catalogManager: CatalogManager)
   private def supportsV1Command(catalog: CatalogPlugin): Boolean = {
     isSessionCatalog(catalog) && (
       SQLConf.get.getConf(SQLConf.V2_SESSION_CATALOG_IMPLEMENTATION).isEmpty ||
-        catalog.isInstanceOf[DelegatingCatalogExtension])
+        catalog.isInstanceOf[CatalogExtension])
   }
 }

sql/core/src/main/scala/org/apache/spark/sql/internal/DataFrameWriterImpl.scala

@@ -429,7 +429,7 @@ final class DataFrameWriterImpl[T] private[sql](ds: Dataset[T]) extends DataFram
     val canUseV2 = lookupV2Provider().isDefined || (df.sparkSession.sessionState.conf.getConf(
         SQLConf.V2_SESSION_CATALOG_IMPLEMENTATION).isDefined &&
         !df.sparkSession.sessionState.catalogManager.catalog(CatalogManager.SESSION_CATALOG_NAME)
-          .isInstanceOf[DelegatingCatalogExtension])
+          .isInstanceOf[CatalogExtension])
 
     session.sessionState.sqlParser.parseMultipartIdentifier(tableName) match {
       case nameParts @ NonSessionCatalogAndIdentifier(catalog, ident) =>