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(ReservedRolesStore.ALERTS_INDEX_ALIAS).privileges("all").build(),
                
                
                RoleDescriptor.IndicesPrivileges.builder().indices(ReservedRolesStore.PREVIEW_ALERTS_INDEX_ALIAS).privileges("all").build(),
                
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(ReservedRolesStore.PREVIEW_ALERTS_BACKING_INDEX_ALIAS)
                    .privileges("all")
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder().indices("metrics-endpoint.policy-*").privileges("read").build(),
                
                RoleDescriptor.IndicesPrivileges.builder().indices("metrics-endpoint.metrics-*").privileges("read").build(),
                
                RoleDescriptor.IndicesPrivileges.builder().indices("logs-endpoint.events.*").privileges("read").build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(
                        "logs-*",
                        "synthetics-*",
                        "traces-*",
                        "/metrics-.*&~(metrics-endpoint\\.metadata_current_default.*)/",
                        ".logs-endpoint.action.responses-*",
                        ".logs-endpoint.diagnostic.collection-*",
                        ".logs-endpoint.actions-*",
                        ".logs-osquery_manager.actions-*",
                        ".logs-osquery_manager.action.responses-*",
                        "profiling-*"
                    )