Add toString for ProjectStateRegistry (elastic#130660)

It is helpful to see the actual content as part of ClusterState#toString
output. Such output is sometimes logged when test fails and useful for
troubleshooting.

Author: ywangd

server/src/main/java/org/elasticsearch/cluster/project/ProjectStateRegistry.java

@@ -145,6 +145,18 @@ Map<ProjectId, Settings> getProjectsSettings() {
         return Collections.unmodifiableMap(projectsSettings);
     }
 
+    @Override
+    public String toString() {
+        return "ProjectStateRegistry["
+            + "projectsSettings="
+            + projectsSettings
+            + ", projectsMarkedForDeletion="
+            + projectsMarkedForDeletion
+            + ", projectsMarkedForDeletionGeneration="
+            + projectsMarkedForDeletionGeneration
+            + ']';
+    }
+
     @Override
     public boolean equals(Object o) {
         if (this == o) return true;