Use CrossProjectModeDecider in RemoteClusterService

Addresses a TODO in RemoteClusterService by using the
CrossProjectModeDecider which handles the details of
the settings lookup.  This is the class used throughout
the codebase now to test if CPS is enabled.

Author: JeremyDahlgren

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

@@ -33,6 +33,7 @@
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.indices.IndicesExpressionGrouper;
 import org.elasticsearch.node.ReportingService;
+import org.elasticsearch.search.crossproject.CrossProjectModeDecider;
 
 import java.io.Closeable;
 import java.io.IOException;
@@ -80,7 +81,7 @@ public boolean isRemoteClusterServerEnabled() {
     private final Map<ProjectId, Map<String, RemoteClusterConnection>> remoteClusters;
     private final RemoteClusterCredentialsManager remoteClusterCredentialsManager;
     private final ProjectResolver projectResolver;
-    private final boolean crossProjectEnabled;
+    private final CrossProjectModeDecider crossProjectModeDecider;
 
     RemoteClusterService(Settings settings, TransportService transportService, ProjectResolver projectResolver) {
         super(settings);
@@ -97,11 +98,7 @@ public boolean isRemoteClusterServerEnabled() {
         if (remoteClusterServerEnabled) {
             registerRemoteClusterHandshakeRequestHandler(transportService);
         }
-        /*
-         * TODO: This is not the right way to check if we're in CPS context and is more of a temporary measure since
-         *  the functionality to do it the right way is not yet ready -- replace this code when it's ready.
-         */
-        this.crossProjectEnabled = settings.getAsBoolean("serverless.cross_project.enabled", false);
+        this.crossProjectModeDecider = new CrossProjectModeDecider(settings);
         // Since this counter may never be incremented we need to force an initial observed value of zero via add(0) so the metric will be
         // in the mappings of the indices of the APM data stream, otherwise we will encounter 'not found' errors in dashboards and alerts.
         // See observability-dev issue #3042.
@@ -232,15 +229,15 @@ void ensureConnected(String clusterAlias, ActionListener<Void> listener) {
      * it returns an empty value where we default/fall back to true.
      */
     public Optional<Boolean> isSkipUnavailable(String clusterAlias) {
-        if (crossProjectEnabled) {
+        if (crossProjectModeDecider.crossProjectEnabled()) {
             return Optional.empty();
         } else {
             return Optional.of(getRemoteClusterConnection(clusterAlias).isSkipUnavailable());
         }
     }
 
     public boolean crossProjectEnabled() {
-        return crossProjectEnabled;
+        return crossProjectModeDecider.crossProjectEnabled();
     }
 
     /**
@@ -368,6 +365,7 @@ public synchronized void updateRemoteCluster(
         boolean forceRebuild,
         ActionListener<RemoteClusterConnectionStatus> listener
     ) {
+        final var crossProjectEnabled = crossProjectModeDecider.crossProjectEnabled();
         final var projectId = config.originProjectId();
         final var clusterAlias = config.linkedProjectAlias();
         final var connectionMap = getConnectionsMapForProject(projectId);