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
"manage_index_templates",
                        "manage_ingest_pipelines",
                        "monitor",
                        GetLifecycleAction.NAME,
                        PutLifecycleAction.NAME,
                        "cluster:monitor/xpack/watcher/watch/get",
                        "cluster:admin/xpack/watcher/watch/put",
                        "cluster:admin/xpack/watcher/watch/delete" },
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder().indices(".monitoring-*").privileges("all").build(),
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices("metricbeat-*")
                            .privileges("index", "create_index", "view_index_metadata", IndicesAliasesAction.NAME, RolloverAction.NAME)
                            .build() },
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA
                )
            )
            .put(
                "remote_monitoring_collector",
                new RoleDescriptor(
                    "remote_monitoring_collector",
                    new String[] { "monitor" },
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder().indices("*").privileges("monitor").allowRestrictedIndices(true).build(),
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices(".kibana*")
                            .privileges("read")
                            .allowRestrictedIndices(true)
                            .build() },