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
RoleDescriptor.IndicesPrivileges.builder()
                    .indices(".kibana*", ".reporting-*")
                    .privileges("all")
                    .allowRestrictedIndices(true)
                    .build(),
                RoleDescriptor.IndicesPrivileges.builder().indices(".monitoring-*").privileges("read", "read_cross_cluster").build(),
                RoleDescriptor.IndicesPrivileges.builder().indices(".management-beats").privileges("create_index", "read", "write").build(),
                
                RoleDescriptor.IndicesPrivileges.builder().indices(".ml-anomalies*", ".ml-stats-*").privileges("read").build(),
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(".ml-annotations*", ".ml-notifications*")
                    .privileges("read", "write")
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(".apm-agent-configuration")
                    .privileges("all")
                    .allowRestrictedIndices(true)
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(".apm-custom-link")
                    .privileges("all")
                    .allowRestrictedIndices(true)
                    .build(),
                
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(".apm-source-map")
                    .privileges("all")
                    .allowRestrictedIndices(true)