Add Virtual Shards routing and mapping overrides

This PR adds the initial routing and metadata groundwork for virtual shards.

When enabled, routing now uses a virtual shard id (vShardId) before resolving to a physical shard. This separates hash-space size from physical shard count and prepares the path for future shard movement workflows.

What’s included

- New static index setting: index.number_of_virtual_shards (default -1, disabled).
- Routing update in OperationRouting.generateShardId:
    - virtual-shard path when enabled,
    - existing behavior unchanged when disabled.
- New VirtualShardRoutingHelper for vShardId -> physical shard resolution.
- Optional per-index override support via virtual_shards_routing custom metadata.
- Test coverage for:
    - enabled/disabled routing behavior,
    - validation rules,
    - override and fallback behavior.

Out of scope

- Side-car segment extraction flow.
- Transport/state-orchestration for managing override lifecycle.

Author: atris

server/src/test/java/org/opensearch/common/settings/ConsistentSettingsServiceTests.java

@@ -36,8 +36,12 @@
 import org.opensearch.cluster.ClusterStateUpdateTask;
 import org.opensearch.cluster.service.ClusterService;
 import org.opensearch.test.OpenSearchTestCase;
+import org.junit.Assume;
 import org.junit.Before;
 
+import javax.crypto.SecretKeyFactory;
+
+import java.security.NoSuchAlgorithmException;
 import java.util.Arrays;
 import java.util.Locale;
 import java.util.concurrent.atomic.AtomicReference;
@@ -55,6 +59,12 @@ public class ConsistentSettingsServiceTests extends OpenSearchTestCase {
 
     @Before
     public void init() throws Exception {
+        try {
+            SecretKeyFactory.getInstance("PBKDF2WithHmacSHA512");
+        } catch (NoSuchAlgorithmException e) {
+            Assume.assumeNoException("PBKDF2WithHmacSHA512 algorithm is not available", e);
+        }
+
         clusterState.set(ClusterState.EMPTY_STATE);
         clusterService = mock(ClusterService.class);
         Mockito.doAnswer((Answer) invocation -> { return clusterState.get(); }).when(clusterService).state();