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
+ roleDescriptor.getName()
                        + "] must have the same name as the service account ["
                        + id.asPrincipal()
                        + "]"
                );
            }
            this.user = new User(
                id.asPrincipal(),
                Strings.EMPTY_ARRAY,
                "Service account - " + id,
                null,
                Map.of("_elastic_service_account", true),
                true
            );
        }
        @Override
        public ServiceAccountId id() {
            return id;
        }
        @Override
        public RoleDescriptor roleDescriptor() {
            return roleDescriptor;
        }
        @Override
        public User asUser() {
            return user;
        }
    }
}