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/qa/service-account/src/javaRestTest/java/org/elasticsearch/xpack/security/authc/service/ServiceAccountIT.java
"allow_restricted_indices": false
                },
                {
                  "names": [
                    "profiling-*"
                  ],
                  "privileges": [
                    "read",
                    "write",
                    "auto_configure"
                  ],
                  "allow_restricted_indices": false
                },
                {
                  "names": [
                    "traces-apm.sampled-*"
                  ],
                  "privileges": [
                    "read",
                    "monitor",
                    "maintenance"
                  ],
                  "allow_restricted_indices": false
                },
                {
                  "names": [
                    ".fleet-*"
                  ],
                  "privileges": [
                    "read",