feat: Expose autogenerated GAPIC client via getNewApi()

This change adds a `getNewApi()` method to `BigtableTableAdminClient` which returns the underlying `BaseBigtableTableAdminClient`.

By using composition, we expose the fully generated GAPIC client to users, allowing them to access the newest features and proto-based methods. This maintains the legacy client as a backward-compatible facade without risking method signature collisions.

This addresses the legacy client integration phase of the Admin API GAPIC upgrade.

Tracking Bug: b/475818901

Author: jinseopkim0

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/BigtableTableAdminClient.java

@@ -151,6 +151,7 @@ public final class BigtableTableAdminClient implements AutoCloseable {
   private final EnhancedBigtableTableAdminStub stub;
   private final String projectId;
   private final String instanceId;
+  private final BaseBigtableTableAdminClient newApi;
 
   /** Constructs an instance of BigtableTableAdminClient with the given project and instance IDs. */
   public static BigtableTableAdminClient create(
@@ -190,6 +191,7 @@ private BigtableTableAdminClient(
     this.projectId = projectId;
     this.instanceId = instanceId;
     this.stub = stub;
+    this.newApi = BaseBigtableTableAdminClient.create(stub);
   }
 
   /** Gets the project ID of the instance whose tables this client manages. */
@@ -202,6 +204,14 @@ public String getInstanceId() {
     return instanceId;
   }
 
+  /**
+   * Returns the underlying autogenerated GAPIC client.
+   * This provides access to the newest features and proto-based methods.
+   */
+  public BaseBigtableTableAdminClient getNewApi() {
+    return newApi;
+  }
+
   @Override
   public void close() {
     stub.close();