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/security/src/main/java/org/elasticsearch/xpack/security/authc/service/ElasticServiceAccounts.java
"logs-*",
                        "metrics-*",
                        "traces-*",
                        ".logs-endpoint.diagnostic.collection-*",
                        ".logs-endpoint.action.responses-*"
                    )
                    .privileges("write", "create_index", "auto_configure")
                    .build(),
                RoleDescriptor.IndicesPrivileges.builder().indices("profiling-*").privileges("read", "write", "auto_configure").build(),
                RoleDescriptor.IndicesPrivileges.builder()
                    
                    
                    
                    
                    
                    .indices("traces-apm.sampled-*")
                    .privileges("read", "monitor", "maintenance")
                    .build(),
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(".fleet-*")
                    
                    .privileges("read", "write", "monitor", "create_index", "auto_configure", "maintenance")
                    .allowRestrictedIndices(true)
                    .build(),
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices("synthetics-*")
                    
                    .privileges("read", "write", "create_index", "auto_configure")
                    .allowRestrictedIndices(false)
                    .build() },