Part of https://github.com/elastic/kibana/issues/154715

The `.fleet-secrets` system index should just have a value key. 

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only. 

Example document:

```
// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}
```
 
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/store/ReservedRolesStore.java
public static final String ALERTS_LEGACY_INDEX = ".siem-signals*";
    
    public static final String ALERTS_BACKING_INDEX = ".internal.alerts*";
    
    public static final String ALERTS_INDEX_ALIAS = ".alerts*";
    
    public static final String PREVIEW_ALERTS_INDEX_ALIAS = ".preview.alerts*";
    
    public static final String PREVIEW_ALERTS_BACKING_INDEX_ALIAS = ".internal.preview.alerts*";
    
    public static final String LISTS_INDEX = ".lists-*";
    
    public static final String LISTS_ITEMS_INDEX = ".items-*";
    public static final RoleDescriptor SUPERUSER_ROLE_DESCRIPTOR = new RoleDescriptor(
        "superuser",
        new String[] { "all" },
        new RoleDescriptor.IndicesPrivileges[] {
            RoleDescriptor.IndicesPrivileges.builder().indices("*").privileges("all").allowRestrictedIndices(false).build(),
            RoleDescriptor.IndicesPrivileges.builder()
                .indices("*")
                .privileges("monitor", "read", "view_index_metadata", "read_cross_cluster")
                .allowRestrictedIndices(true)
                .build() },
        new RoleDescriptor.ApplicationResourcePrivileges[] {
            RoleDescriptor.ApplicationResourcePrivileges.builder().application("*").privileges("*").resources("*").build() },