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
.build(),
                
                RoleDescriptor.IndicesPrivileges.builder().indices("apm-*").privileges("read", "read_cross_cluster").build(),
                RoleDescriptor.IndicesPrivileges.builder().indices("logs-apm.*").privileges("read", "read_cross_cluster").build(),
                RoleDescriptor.IndicesPrivileges.builder().indices("metrics-apm.*").privileges("read", "read_cross_cluster").build(),
                RoleDescriptor.IndicesPrivileges.builder().indices("traces-apm.*").privileges("read", "read_cross_cluster").build(),
                RoleDescriptor.IndicesPrivileges.builder().indices("traces-apm-*").privileges("read", "read_cross_cluster").build(),
                
                RoleDescriptor.IndicesPrivileges.builder().indices("*").privileges("view_index_metadata", "monitor").build(),
                
                RoleDescriptor.IndicesPrivileges.builder().indices(".logs-endpoint.diagnostic.collection-*").privileges("read").build(),
                
                
                RoleDescriptor.IndicesPrivileges.builder().indices(".fleet*").allowRestrictedIndices(true).privileges("all").build(),
                
                RoleDescriptor.IndicesPrivileges.builder().indices("logs-elastic_agent*").privileges("read").build(),
                
                
                RoleDescriptor.IndicesPrivileges.builder().indices(ReservedRolesStore.ALERTS_LEGACY_INDEX).privileges("all").build(),
                
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(ReservedRolesStore.LISTS_INDEX, ReservedRolesStore.LISTS_ITEMS_INDEX)
                    .privileges("all")
                    .build(),
                
                
                RoleDescriptor.IndicesPrivileges.builder().indices(ReservedRolesStore.ALERTS_BACKING_INDEX).privileges("all").build(),