[SPARK-49147][CORE] Mark KryoRegistrator with DeveloperApi interface

### What changes were proposed in this pull request?
Trait `org.apache.spark.serializer.KryoRegistrator` is a public interface because it is exposed via config `spark.kryo.registrator`, but it is not annotated to describe the stability level. This adds the `DeveloperApi` annotation to formalize the contract.

### Why are the changes needed?
This will help users understand the compatibility guarantees when using the trait. It is also helpful for scripts that need to identify classes with public APIs. We recently missed this trait in a script used to analyze which namespaces can be shaded.

### Does this PR introduce _any_ user-facing change?
Yes, it adds the DeveloperApi interface to the trait.

### How was this patch tested?
N/A. Annotation change only.

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

Closes #47657 from robreeves/rob/stable_kryo.

Lead-authored-by: Rob Reeves <[email protected]>
Co-authored-by: Kent Yao <[email protected]>
Signed-off-by: Kent Yao <[email protected]>

core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala

@@ -38,6 +38,7 @@ import org.apache.avro.generic.{GenericContainer, GenericData, GenericRecord}
 import org.roaringbitmap.RoaringBitmap
 
 import org.apache.spark._
+import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.api.python.PythonBroadcast
 import org.apache.spark.internal.{Logging, MDC}
 import org.apache.spark.internal.LogKeys.CLASS_NAME
@@ -519,6 +520,7 @@ private[spark] class KryoSerializerInstance(
  * Interface implemented by clients to register their classes with Kryo when using Kryo
  * serialization.
  */
+@DeveloperApi
 trait KryoRegistrator {
   def registerClasses(kryo: Kryo): Unit
 }