Fix S3ClientSettings Class Loading (#66886)

This is motivated by the inability to run
`org.elasticsearch.repositories.encrypted.EncryptedS3BlobStoreRepositoryIntegTests`
in isolation without this workaround. The way integration tests load classes
otherwise leads to a load order which doesn't load the plugin class first,
thus fails to apply the jackson workaround before further S3 classes are loaded
but depend on our Jackson workaround.

Author: original-brownbear

plugins/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3ClientSettings.java

@@ -41,6 +41,16 @@
  */
 final class S3ClientSettings {
 
+    static {
+        // Make sure repository plugin class is loaded before this class is used to trigger static initializer for that class which applies
+        // necessary Jackson workaround
+        try {
+            Class.forName("org.elasticsearch.repositories.s3.S3RepositoryPlugin");
+        } catch (ClassNotFoundException e) {
+            throw new AssertionError(e);
+        }
+    }
+
     // prefix for s3 client settings
     private static final String PREFIX = "s3.client.";