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
RoleDescriptor.IndicesPrivileges.builder().indices("logs-apm.*").privileges("read", "view_index_metadata").build(),
                        RoleDescriptor.IndicesPrivileges.builder().indices("logs-apm-*").privileges("read", "view_index_metadata").build(),
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices("metrics-apm.*")
                            .privileges("read", "view_index_metadata")
                            .build(),
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices("metrics-apm-*")
                            .privileges("read", "view_index_metadata")
                            .build(),
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices("traces-apm.*")
                            .privileges("read", "view_index_metadata")
                            .build(),
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices("traces-apm-*")
                            .privileges("read", "view_index_metadata")
                            .build(),
                        
                        
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices(".ml-anomalies*")
                            .privileges("read", "view_index_metadata")
                            .build(),
                        
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices("observability-annotations")
                            .privileges("read", "view_index_metadata")
                            .build() },