Add Provider interface for LinkedProjectConfigService (#136310)

Adds a Provider interface that returns an Optional
LinkedProjectConfigService, to support optionally
creating an instance from serverless, depending on
if CPS is enabled.

Relates: ES-12270

Author: JeremyDahlgren

server/src/main/java/org/elasticsearch/node/NodeConstruction.java

@@ -1009,9 +1009,11 @@ public Map<String, String> queryFields() {
         final IndexingPressure indexingLimits = new IndexingPressure(settings);
 
         final var linkedProjectConfigService = pluginsService.loadSingletonServiceProvider(
-            LinkedProjectConfigService.class,
-            () -> new ClusterSettingsLinkedProjectConfigService(settings, clusterService.getClusterSettings(), projectResolver)
-        );
+            LinkedProjectConfigService.Provider.class,
+            () -> Optional::empty
+        )
+            .create()
+            .orElseGet(() -> new ClusterSettingsLinkedProjectConfigService(settings, clusterService.getClusterSettings(), projectResolver));
 
         PluginServiceInstances pluginServices = new PluginServiceInstances(
             client,

server/src/main/java/org/elasticsearch/transport/LinkedProjectConfigService.java

@@ -10,12 +10,24 @@
 package org.elasticsearch.transport;
 
 import java.util.Collection;
+import java.util.Optional;
 
 /**
  * Service for registering {@link LinkedProjectConfigListener}s to be notified of changes to linked project configurations.
  */
 public interface LinkedProjectConfigService {
 
+    /**
+     * Interface for providing a {@link LinkedProjectConfigService} instance via SPI.
+     */
+    interface Provider {
+        /**
+         * @return An {@link Optional} populated with a {@link LinkedProjectConfigService} instance, or {@link Optional#empty()} if it is
+         * not possible to create an instance in the current runtime environment.
+         */
+        Optional<LinkedProjectConfigService> create();
+    }
+
     /**
      * Listener interface for receiving updates about linked project configurations.
      * Implementations must not throw from any of the interface methods.