Reload KnnVectorsFormat in Lucene SPI registry. (elastic#129197)

In order for plugins to extend Lucene's KnnVectorsFormat, we need to reload
it along with other Lucene SPIs when the plugin classloader is created.

This commit adds the call to KnnVectorsFormat.reloadKnnVectorsFormat(loader)
in reloadLuceneSPI method, allowing full extension of Lucene's vector functionality through plugins.

Author: Rassyan

server/src/main/java/org/elasticsearch/plugins/PluginsService.java

@@ -13,6 +13,7 @@
 import org.apache.logging.log4j.Logger;
 import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.codecs.DocValuesFormat;
+import org.apache.lucene.codecs.KnnVectorsFormat;
 import org.apache.lucene.codecs.PostingsFormat;
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.admin.cluster.node.info.PluginsAndModules;
@@ -477,6 +478,7 @@ static void reloadLuceneSPI(ClassLoader loader) {
         // Codecs:
         PostingsFormat.reloadPostingsFormats(loader);
         DocValuesFormat.reloadDocValuesFormats(loader);
+        KnnVectorsFormat.reloadKnnVectorsFormat(loader);
         Codec.reloadCodecs(loader);
     }