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
null,
                    null,
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA,
                    null
                )
            )
            .put(
                "ingest_admin",
                new RoleDescriptor(
                    "ingest_admin",
                    new String[] { "manage_index_templates", "manage_pipeline" },
                    null,
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA
                )
            )
            
            .put(
                "reporting_user",
                new RoleDescriptor(
                    "reporting_user",
                    null,
                    null,
                    null,
                    null,
                    null,
                    MetadataUtils.getDeprecatedReservedMetadata("Please use Kibana feature privileges instead"),
                    null
                )